The photograph of the Falling Man came to symbolise the unspeakable horrors of 9/11. While written testimony has the capacity to shock, a single iconic image has the power to pack an emotional punch that lingers on in the mind. Graeme Green examines the impact of some of the most shocking and memorable images of recent times

"IT hit her hard when she first saw it, the day after, in the newspaper. The man headlong, the towers behind him The man with blood on his shirt, she thought, or burn marks, and the effect of the columns behind him, the composition, she thought, darker stripes for the nearer tower, the north, lighter for the other, and the mass, the immensity of it, and the man set almost precisely between the rows of darker and lighter stripes. Headlong, free fall, she thought, and this picture burned a hole in her mind and heart, dear God, he was a falling angel and his beauty was horrific."

The horrific beauty, described by Don DeLillo in his latest novel Falling Man, is Associated Press photographer Richard Drew's picture from September 11, 2001, showing one of the estimated 200 people who jumped from the World Trade Center towers to escape the smoke and fire. The unknown man was framed almost symmetrically, set against white and black lines of steel and glass. He seemed to be falling with a calm acceptance of his fate, a heroic, graceful state that defied the terror.

The photo ran once in many American newspapers. It was angrily criticised by those who felt it was exploitative and disrespectful, an accusation, as Esquire's Tom Junod wrote in his The Falling Man story, that it "turned tragedy into leering pornography". Many publications subsequently vetoed it. There was anger, too, at what some saw as the picture's misrepresentation of reality. Drew had taken a 12-shot sequence but only in one of them did the man appear composed. The rest of the frames show him, like the others, flailing fearfully on his way down.

It's a strange image: arresting, oddly intimate, disturbing. Drew believes many were upset by the picture because it made it easy for them to imagine themselves in that same situation. "We saw lots of pictures of planes hitting the buildings, the buildings on fire, the buildings falling down," says Drew. "I'm not being flippant, but watching the buildings come down over and over again was very much like watching some kind of Hollywood special effects. We saw very little of the people. The Falling Man picture is something everyone can identify with. I think it's hard for people to look at because they can identify with it too much."

Who was the Falling Man so many came to identify with? No one knows. And, most likely, no one ever will. Writers and reporters tried unsuccessfully to establish his identity, to locate friends or relatives. There were several claims and possible matches, but nothing conclusive. The photos show a man, probably black or Latino, possibly with a goatee, wearing black shoes and an orange T-shirt, but digital enhancement is unable to tell us any more. He's likely to remain anonymous.

As such, the Falling Man has become a blank face on to which many have written their own thoughts and feelings. For some, he symbolises the grief and fear, the human cost of the attack. For others, he kindled feelings of anger and vengefulness. Others still took hope in his apparent steeliness, the triumph of the human spirit in desperate times. Whoever he was, he now stands for something much larger than his own story. He's the innocent attacked, the vulnerability of the West, the start of future wars. The still moment before the world changed.

It's not uncommon for one anonymous individual captured on film to go on to speak' for all of those like them, the hundreds or thousands caught up in terrorist attacks or wars, exoduses, famines, natural disasters. They can become potent symbols.

Washington Post photojournalist Lucian Perkins won the World Press Photo of the Year award in 1995 for his picture of a young boy in Chechnya peering out of a bus loaded with refugees as they fled fighting between Chechnyan and Russian forces. Perkins never found out anything about the boy. But, he says, the boy's identity isn't important.

"I just happened to capture the image of this one kid, but you know there are a lot more kids like him," he says, "some that have died and others who've had horrible things happen to them. That you're able to capture something that has happened to a lot of people, I think is very important."

The same could be said of Kevin Carter's 1994 picture from Sudan: an emaciated toddler on her way to a feeding centre, a watchful vulture in the background. Appearing on newspaper covers around the world, the nameless girl became a powerful symbol for all the children suffering in Africa's famine. There are others too: James Nachtwey's picture of a Hutu man, his face scarred by the violence in Rwanda; Harry Benson's picture of an old woman angrily banging a rock against the Berlin Wall.

The most famous of all these nameless icons, perhaps, is the individual who, in 1989, stepped into Tiananmen Square and stopped an advancing line of tanks. His identity never revealed, he's known only as the Unknown Rebel or Tank Man. AP photographer Jeff Widener fought through bloody streets, narrowly avoiding angry mobs of protesters and Chinese police carrying electric cattle-prods, borrowing film from tourists in the hotel below to get the iconic shot.

The striking image ("like a tiny little ant against a buffalo", says Widener) went on to hit the front page of the world's newspapers, focusing attention on the plight of protesters and the atrocities of the Tiananmen massacre. It was immediately seized upon by political groups, students and human rights activists, all claiming Tank Man as their own.

Even the Chinese government tried to make the image talk for them, arguing it showed how humanely they (who killed an estimated 3000-7000 civilians in order to crush the uprising) had treated the lone man by not riding over him with the tank.

The picture continues to grow in stature. On the internet, groups have adopted the image or turned Tank Man into cartoons or logos to stand for their cause. Bands have written songs about him. The idea of a lone man stopping tanks in their tracks, like a modern-day David vs Goliath, is attractive to anyone who wants to believe one person can make a difference, that sometimes the small guy' wins. "Tank Man can apply to many things," says Widener, "political, human effort, triumph over cancer whatever. It can represent triumph over anything major, or just simply hope or belief. It means a lot to different people."

There's been great speculation about who this inspirational figure was and what happened to him: that he was arrested on the spot and disappeared', that he was executed months later, that he is still alive and hiding in China. "I think some day we might know who he is," says Widener. "I think there are individuals who know who is - his family or friends. But it's almost like he was destined to be this way."

There's something particularly resonant about anonymous heroes like Tank Man. Unlike celebrities that we read so much about, destroying any mystery about them, when it comes to anonymous figures, the facts aren't so neatly tied up. Just as the Marie Celeste would become just another boat if we knew what really happened or the Loch Ness monster if caught would become just another sea creature, there's a freedom in not knowing. Mysteries stay with us far longer and make us think far more - we're free to speculate and imagine.

Might it be better if we never find out who Tank Man is? "If I had the choice to meet this man and say, What you did was an amazing thing,' I would," says Widener. "But it's like if they finally found out who popped off Jimmy Hoffa - it would blow the mystique of that whole thing. I mean, Tank Man is the faceless man. It makes him even more representative to different people and their beliefs. I can see that staying unknown is something a lot of groups might want."

Even when the identity of a person in a picture is known, photographic icons often continue to have a significance beyond just their own life. We know the name of the monk who, in 1963, burned himself to death in Saigon to protest against the South Vietnamese President Ngo Dinh Diem's oppression of the Buddhist faith, was Thich Quang Duc. But through Pulitzer prize-winning photographer Malcolm W Browne's harrowing picture, he became known simply as the Burning Monk, a symbol of the extremes to which people will go to fight for their human rights. The subject of photojournalist Dorothea Lange's 1936 picture (a mother, camped beside an American road, holding her children) was, 40 years later, identified as Florence Owens Thompson, but she'll continue to be more commonly known as Migrant Mother, the embodiment of the poverty caused by the Great Depression.

One of the most enduring pictures from history shows a nine-year-old girl running down a road, fleeing a napalm attack during the Vietnam War. She's naked, having torn off her clothes, which were on fire. AP photographer Nick Ut, who won the Pulitzer for the picture, put the camera down immediately after shooting to pick the girl up and take her to get medical help. "I didn't want her to die," he recalls, 35 years later.

The girl was Kim Phuc. She survived the war and now lives in Canada. But she's known far more for the image, seared on to the public consciousness as Napalm Girl or Running Girl, than for her own name. "Kim Phuc became a picture for all children in the Vietnam war, the children dying every day," says Ut. "Her picture humanised the pain of war."

More recently, Reuters photographer Arko Datta's pictures of the Boxing Day tsunami of 2004 appeared on the front pages of newspapers and magazines around the world. His picture of an Indian woman on a beach mourning the death of a relative, in particular - her face pained, palms turned upwards, the arm of a corpse just in frame - brought home the horror and suffering of the disaster, far more powerfully than statistics or pictures of giant waves could. Few would know her by her name: Indira Mariyappam. She was just one of thousands who had lost loved ones in the tsunami.

"I met her later," says Datta. "We learned her name, her age. But, really, her identity in that situation was not important. It was more of what she represented and stood for, for everyone else. It wasn't just her story."

The photojournalist's relationship to their subject is rarely detached or cold. Nick Ut got involved and saved a life. Arko Datta was so emotionally involved during the tsunami he struggled to take pictures at all. Kevin Carter couldn't leave behind the images of what he'd seen and photographed. Though he chased the vulture away that day in Sudan, the guilt he felt and the criticism he received for not helping the starving girl, alongside the atrocities he'd witnessed over the years was, many believe, a significant factor in his suicide. The note he left spoke of "vivid memories of killings & corpses & anger & pain... of starving or wounded children, of trigger-happy madmen, often police, of killer executioners".

What good can these pictures do? Can they affect how people see and think about the world? Are they powerful enough even to change the course of world events? "I think so," says Andy Steel, author of Photojournalism: The World's Top Photographers. "One of my favourite shots is by Robert Capa, from the Spanish Civil War, of a soldier as he's being shot. It had a massive influence on people's thoughts about going to war and showed the reality to people. The Tank Man picture, too, is a wonderful gilt-edged example of how pictures can be used to influence people around the world and show these things are wrong and how we have to do something about it."

Jeff Widener agrees. "Pictures throughout the years have changed things dramatically. You look at Joe Rosenthal's flag-raising on Iwo Jima - that had a major impact on the war, on the war machine and raising bonds. Still images are very powerful. You can read some great writing and think about it and imagine it, but a picture sometimes is so brutal. With my Tank Man picture, it's hard to say but had the picture not been seen worldwide, maybe things in that situation would have been even worse."

Nick Ut is happy to take credit, too, for helping stop or at least slow the spread of violence. "Even today, people tell me my picture is part of the history of Vietnam," he says. "I've met so many American veterans who fought in Vietnam, who've said, Nicky, thank you so much for your pictures, because your pictures stopped the Vietnam war'."

Such is the understood power of pictures that they're often suppressed. In China, pictures of the Tiananmen Square massacre have been effectively censored by the great firewall of China', the authorities' restrictive system of online blocks and filters. And it is widely accepted media images of the Vietnam war turned American opinion against the conflict, which is why, says Richard Drew, America is keen to restrict reporting on Iraq.

"The US army is very eager to not allow images of injured servicemen," he says. "And the Pentagon doesn't want us photographing the caskets of the dead coming back to the United States. You see a picture of caskets coming home, it's going to leave a more lasting impression than hearing about how many people got killed in a bomb attack."

The most shocking and damaging images of recent years were the leaked pictures of prisoner abuse in Abu Ghraib: piles of naked humiliated men, death's angel' figures stood on boxes with masks over their heads and electric wires connected to their fingers.

"When one sees evidence like the pictures of Abu Ghraib, it affects world opinion," says Arko Datta. "People who see these pictures as they have their breakfast, in their comfortable lives, they have to take notice. These pictures force them to look beyond their lives."

Not every photographer has such firm faith, though. "It's a question I always ask: do photographs make a difference?" says Lucian Perkins. "I remember during the war in Bosnia, a number of photographers were risking their lives every day and they said, It seems like we're not making a difference at all'. I grew up during the Vietnam war and I remember seeing the images and thinking, I hope we never do anything like that again. Now we're in Iraq and you have to think, maybe photographs didn't help stop this.

"With my photograph of Chechnya," he continues, "a lot of people said it made an impact on them, made them aware there was a war in some faraway place called Chechnya that they knew very little about. But as a photographer, personally, I had to get rid of the notion that any of our photographs are going to change the world. Perhaps they can be one piece of evidence that causes change over time. But what's frustrating is you wish they would have more of an effect."

There are times when the effect of a photograph is clearer though. Arko Datta's memorable picture of the woman grieving after the tsunami, for example, no doubt helped fuel the massive fundraising and aid relief that followed - more than $7 billion in public and government donations from around the world. "From the feedback I got, people were looking at the picture, thinking about it and reacting to it," he says. "If the media wasn't there, I don't think people would even have known what really happened. I played a part and I feel very satisfied by that."

Jeff Widener, too, now focusing his lens on local projects in Hawaii, has seen tangible results from his work. "I did a story on a children's hospital in Bangladesh after a cyclone and someone donated $25,000 after seeing my images. I mean, that makes you feel great. In Hawaii, we've got people moved into transitional housing because of a story we did on Hawaii's homeless problem. I'm not saying it's going to change the world, but at least let's show another side and get people to open their eyes a little bit."

What happens once pictures are in the public domain, whether they stop wars, raise money or become the subject of contentious debate, is out of the photographer's hands. The only duty of the photojournalist, says Michiel Munneke, director of the World Press Photo Foundation, is to record. "It's a kind of testimonial. It's about recording events that shouldn't be forgotten."

What they always do, in recording these events, is increase awareness, hooking us by the eyes and drawing us into stories that wouldn't impact so greatly or perhaps wouldn't register at all if communicated only with words. These photographed individuals - Tank Man, Falling Man, Burning Monk, Napalm Girl - are often our point of entry into the lives of activists, bomb victims, refugees and prisoners around the world.

Whether we act or not, seeing their pictures makes it impossible not to think of the many others just like them, the living and the dead, who never made it into the frame at all.

Photojournalism: The World's Top Photographers by Andy Steel, Rotovision, £25.

The World Press Photo Exhibition opens at the Scottish Parliament on August 3