To quote a joke e-mail I remember getting manyyears ago...
Mens magazines contain photos of naked women. Women's magazines also contain photos of semi naked women because the male body is ugly and hairy and should never be seen in the light of day...
Seriously, I think it is all about the fetish for photography.
Those subs who post either have a fetish for exhibitionism or Dom/mes who have a fetish for control to the extent that they command the sub to do something they may not be comfortable with. A lot of the self photography threads seem to have titles which reflect this. So, subs post because they are either told to do so (and they are good subs) or because they have a desire (either conscious or subconscious) to be seen naked on the internet. I have personally posted photos online for both these reasons, though always anonymously. To an exhibitionist, the fact that someone can trace the photo back to you or it can come back to haunt you later is actually the point of the exercise.
So, with this argument in mind, what is there to tempt Doms or Dommes to post photos? Who is going to command them? Why would they post photos of themselves when they have an exhibitionist sub to do it for them?