In the four years since the Scottish National Portrait Gallery (SNPG) reopened, it has welcomed into its collection around 50 new artworks.

Some - such as Cecile Walton's huge 1912 painting Eric Robertson (1887-1941), Artist, With Mary Newbery (1890-1985) - are old friends which have been brought into the family on a permanent footing after being gifted to the gallery's collection by descendants of the artist. Walton was the daughter of Glasgow Boys artist Edward Arthur Walton. Elsewhere, a wonderful series of black and white photographs taken in Glasgow in the late 1960s by David Peat were gifted by the artist's family following his death in 2012.

Collecting Now offers up an opportunity to see these newly acquired works arrayed in all their glory. The line-up also includes Christian Hook's quirky tease of a portrait of actor Alan Cumming, first unveiled in December 2014 as part of a collaboration with Sky Arts, and Joyce Gunn Cairns's beautifully tender drawing of poet Edwin Morgan (1920-2010).

According to SNPG director Christopher Baker, the aim of Collecting Now has been to demonstrate the richness and diversity of the collection. "The photography which we are showing is especially strong," he says. "This reflects the fact that photography is an important part of our programming. Since the Portrait Gallery reopened in 2011 we have had a dedicated space for photography which didn't exist before. A number of the works have not been seen before, such as three glorious black and white photographs of Scottish jazz singer Carol Kidd, by the famous American photographer Eve Arnold, who died in 2012."

There is also photographic work on show by photographer David Eustace, who recently held a successful selling show at The Scottish Gallery in Edinburgh, the first exhibition of photography in its long and illustrious history. Eustace gifted several of his own works, including a close-up portrait of Dundonian actor Brian Cox and one of the internationally renowned Edinburgh-born fashion photographer Albert Watson.

Other lens-like treats include work by the late Colin Jarvie (1962-2012). Jarvie was a commercial fashion photographer who emerged from the post-punk music scene of 1979-80. Around this time, he began photographing bands associated with Edinburgh-based independent label, Fast Product. Founded by Bob Last and Hilary Morrison, early releases included singles by The Human League, The Gang Of Four and The Mekons.

After studying photography, film and television at the London College of Printing (now London College of Communication, University of the Arts, London), Jarvie set up Avid Images with David Scheinmann in 1985. He later returned to the London College of Printing, where he taught photography until 2006. In one gelatin silver print portrait taken around 1980, the sweeping construction of a satin hat is playfully echoed in the model's pose, both in the positioning of her arm and the curve of the cigarette holder. Following Jarvie's death in 2012, his family donated works to the National Galleries of Scotland in recognition of his contribution to photography.

Aberdeen-born singing star and campaigner Annie Lennox has also gifted work, including a ghostly collaborative portrait photograph taken in 2003 by Lennox and Allan Martin and used as the album cover of Lennox's third solo studio album, Bare.

As well as the David Peat collection, there is contemporary work by Dutch photographer Viviane Sassen (born 1972), who was the subject of a successful exhibition in the gallery in 2013. Paisley-born photographer Rankin is also represented with a moving portrait of Edinburgh resident Emma Louise (Lou) Page, a charity worker died aged just 43 in 2013 after suffering from bone cancer.

The collection is brought bang up-to-date with a group of four photogravure prints by emerging photographer Alex Boyd. In his Stacashal: Views North, South, East And West from Land Of My Desire series, Boyd depicts different views of a summit on the island of Lewis. Boyd made these photogravures (a photo-mechanical process that dates from 1878) standing beside an ancient chambered burial cairn. He used the points of the compass to help record the views across the moors; Muirneag in the North of the island, and Cailleach na Mòinteach (The Old Woman of the Moors) to the South.

According to Christopher Baker, the Cecile Walton painting is destined to become one of the gallery's most popular pictures. This six-foot-long work links three key figures of the Scottish art scene from the days pre-dating the First World War. Eric Robertson, who became Walton's husband, was a landscape painter who went on to scandalise polite Edinburgh society with what appeared to be a ménage à trois with his wife and another painter called Dorothy Johnstone. The Robertsons' marriage stalled, apparently due to his excessive drinking, and Walton moved in with Johnstone. The painting moved in with them too.

There is a suggestion that Mary Newbery, a close friend of Robertson and Walton, and an artist herself (she was the daughter of Glasgow Girls artist Jessie Newbery and Glasgow School of Art director Fra Newbery) had a similar arrangement.

Walton's painting depicts an idyllic scene, with Robertson and Newbery seated in a luxurious garden. Newbery is shown with a flower crown on her lap, a hint from Walton at the symbolism of flowers which can often be seen in Newbery's work.

"It's a pre-World War One painting about three people on the cusp of grown-up life," says Baker. "They were genuinely friends, but there is a romantic element to it. It's wonderfully lyrical. I think it's a picture which will become very popular. Cecile Walton is a very interesting artist. She was a modernist, with a nod back to old Masters."

Collecting Now, Scottish National Portrait Gallery, Edinburgh (0131 624 6200, www.nationalgalleries.org) until September 20, admission free