While pictures fare slightly better than words, they are often put on the back of packs, making them less visible and less effective, according to research published online in the journal Tobacco Control.
More than 1000 youngsters aged 11 to 16 were questioned about their response to warnings in 2008 and a further 1000 were questioned in 2011. Most of the teenagers in both surveys (68% to 75%) had never smoked, while 17% to 22% had experimented with cigarettes and around one in 10 was a regular smoker.
Half of those questioned in both surveys said they had often or very often noticed the warnings on packs, and around one in five had read them very often or looked closely at them.
The number of teenagers saying the warnings put them off smoking increased between the two surveys, but not among regular smokers.
The teenagers' ability to recall images depicting diseased lungs, rotten teeth and neck cancer, remained below 10% while three text warnings on the back of packs with no supporting images were recalled by less than 1% in either survey.
The authors, from the Centre for Tobacco Control Research at Stirling University, said: "As warnings need to be salient to be effective, positioning pictorial warnings only on the less visible reverse panel limits their impact. It may have had a deterrent effect on experimental smokers, but the impact on regular smokers was negligible."