Sam makes complete sense. That said, some people post pictures because it's a natural thing they like to do -- as much as it might baffle the likes of Martin, etc. It's not always about getting ego strokes. Hey, some people are exhibitionists... or at least like to be on camera.
The worst thing to do is to kill that golden goose, though. If the poster is getting enjoyment out of showing themselves, and a lot of other posters respond with appreciation, nobody's getting hurt. They only get hurt if someone steps in and tries to call the cops on the party, if you catch my drift...