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"). Remember also to get people identified properly, with their full names (not "Uncle Jimmy", as that does not tell whose uncle he is!), and his dates of birth and death if possible, in case the name repeats over the generations. The date when the photo was taken is also vital. By looking at various dated photographs, you can find relatives in them that you can then identify by their apparent age in the picture. Clothes can also help identify the date, as fashion changed quite rapidly. The location also tells you who was where at that time. Check out the background - it may have other clues for you. Formal pictures were taken in studios, with artificial backgrounds and usually a few props as well. Those probably have the photographer's studio name and address somewhere.

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.