Most Helpful Guy
If you two are courting or some such then he either wants sexy pictures as mentioned below, otherwise he may just want a picture to put in his wallet or next to his bed - to remind him of you.
If he likes you the same thing applies.
If your just his friend he may still like you and above applies - or maybe he just wants a picture of you to add to his "pictures of friends" album or something lol.
If you don't know him very well he probably wants to get into your pants and you know what he wants.