Ladies, should I feel strange about this or is it no big deal?

My girlfriend is moving into a new apartment. While I was helping her pack up her bookshelves I came across about 20 photos of her having . Pretty much everything was covered in the photos even some things she won't do with me. Based on the color and length of her hair and lack of a nose piercing in the photos I'm guessing the pictures were taken about three years ago, and probably not too long before we got together. I'm pretty sure I can guess who the guy is in the photos and he is still in her circle of friends. Should this concern me? Why does she keep the photos? Do lots of women let boyfriends take explicit photos of them? Do the women keep them?