In my opinion the first thing you have to think about when you are placing the advertisemnents on your gallery is: "Why are the people clicking on my banners"
I think the answer is: "Because I like these pictures and I want more pictures of the same I already saw in the gallery"
When you know this you can conclude that the you will get the best conversion ratio (and the highest turnover) when you use niche sites that offer the same as the sort pictures on your site!
Conclusion:
lesbian pics -----> lesbian sponsor
amateur pics -----> amateur sponsor (f.e. AP)
Take a look at a few of my galleries and you exactly know what I mean:
Pornstar Pics -----> Pornstar sponsor
http://www.downlinefiesta.com/tgp/sky1/
Amateur Teens -----> Amateur sponsor
http://xxxhotxxx.freepornspace.com/b2/
But when use targetted sponsors you have to take care that your traffic is not closing your site immediately. The best way to achieve this is in my opinion by making clean galleries. Surfers don't like scattered galleries with lots of adds. I believe that clean galleries make more bannerclicks than banner farm galleries. And when this isn't true than you will get a better conversion ratio with this approach, because people are not fucked up because of the shitty gallery they saw. Clean galleries will have bannerclicks of people who want to see wat the sponsor is offering. This will result in a better conversion ratio.