The chances are that Granny has lots of pictures where she knows who is in them, but no-one else does. When she dies, her memories are lost, and the photos become useless. Persuade her to put a caption below each photo, stating who is in the picture (from left to right, and front row, back row, etc. to identify the position of each person).

Try to pin down the date and place of the photograph (e.g. "Outside 23 Vernon St., Glasgow, about June 1947").

DO NOT write directly on the back of a photograph, or you will damage it. Instead, write on a peel-off sticky label, and then stick that on the back of the photo or beneath it if in an album.