As a teacher, I never ask my students for photos. It just seems weird to me. However, many have given them to me over the years, and I have some that I have taken on field trips. I keep them all in a folder and go through them about once a year. They always make me smile. It could be as innocent as that. Like others have said, always pay attention to your gut instincts. However, I would not be concerned over this one thing unless other things happened to add to it and make me uncomfortable. It's really an innocent picture, as well, so that makes me think there is nothing to it.