My first question would be "why in gods green earth are you hosting pictures of naked people"?!
But in any case, images are generally copyright to their owners. In the event of discovery, the normal procedure that I know is that they will most likely either contact you or your site host regarding the issue. Generally they'll either ask that you give them credit or that you take it down. Some may ask for monentary compensation, though they'd probably be hard pressed to get that as far as I know...
As far as taking images of naked people, the very issue is shady in itself. Generally you have to get signed releases to display pictures of people period, let alone display them naked when a friend of theirs did the emailing. If that's the case, I imaging they could sue for defimation of character, etc.
Of course, I may be mistaken in some parts of this.
Best thing to do with these pictures would be to put up a disclaimer that they're not yours and you don't know who the owner is. Also add that if the owner has a problem they can contact you to either have it removed or have the proper credit given.