Paul McCartney treasures a photograph his late wife Linda took of him and John Lennon, as it reminds him they were “lifelong friends”.

The pair met as school kids in Liverpool, England in the late 1950s, and went on to worldwide fame as the central creative force behind The Beatles. However, the duo feuded for several years after the band’s 1970s split, before making up shortly before John’s death in 1980.

Paul’s late wife Linda, who passed away in 1998, was a rock photographer who documented the group at the height of their 1960s fame. And the musician admitted he loves looking at one particular snap she took of him and John working on a song together at Abbey Road Studios in London.

“It wasn’t too long before the breakup of The Beatles; this would be the end of our relationship and, at the end, when the breakup happened, it was kind of sour – very difficult to deal with,” he told The Guardian.

Explaining how the image, which shows them smiling while jotting on a notepad, reminds him that he and John weren’t constantly at odds, he added: “So this picture is a blessing for me. It’s like, this is how we were: this is why we related, or else we couldn’t have collaborated for all that time.

“Just seeing the joy between us here really helped me, because it reminds me that the idea we weren’t friends is rubbish. We were lifelong friends, our relationship was super-special.”

A Linda McCartney Retrospective, featuring an array of her professional shots and a selection of her personal family photographs, opens on 5 July at Kelvingrove Art Gallery in Glasgow, Scotland, and runs until 12 January.