Make sure the sponsors and the pics compliment each other. Don't advertise a granny site with amateur pics or vice versa.
I think you actually get more sign-ups when you use the free content from a site you are advertising. If the surfer likes that content and wants to see more, they know they will find it at your sponsor link. For example, I advertise for Busty Amateurs. When I use non-BA amateur pics that I have the license to, I sell less memberships than when I use the BA content.
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