I don't think it's a difference between guys and girls, some girls will keep enormous amounts of pictures of there ex's also. I believe it's basically just personal interest.
You can look at that in both good and bad ways. He may be obsessed with her and might always compare future women to this one, on the other hand he may recognize she was an important figure in his life and he feels it's his duty to acknowledge that.
Does he say more good things about her or more negative things about her?
Impossible is a word only to be found in the dictionary of fools.
Napoleon I