Everyone so far has spoken truth. A rotator and testing to find results. Use your stats to tell you what works too. See which girls get clicked on most especially from members (so you know how people who will PAY react).
One of the paysites I'm involved with is
http://www.facialabuse.com. The whole theme is the cum shots and the "humiliation" factor. So we started the tour with the big thumbs being cum shots where the girls looked totally disgusted. We had a huge list of variations to test, one of which was to switch the big thumbs to the shot that showed the girl in best and most attractive way. So far that's been the best converting permutation. I think that most any niche is still best served by the prettiest girls. But test and find out what works for you.