Among other sites they have Mr. Skin which, as far as I know, is the only celebrity/nude site that is fully licensed to sell real celeb entertainmnet.
They do provide limited free pics to their affiliates for promotion of the program.
As far as buying your own licensed celebrity content; I don't believe that there is a content provider out there.
Celebrity nudes is a touchy issue and a lot of affiliate sites have been shut down during the past years, simply because they had no license for the pics.
Although flashcash is a great program it depends what you use them for, unfortunately flashcash is a banned sponsor on MANY TGPS and other major programs because the entrap the surfer Juicybucks have always done well for us! as well as Traffic cash gold they have been our main providers of celebrity income in the past year and I highly recommend them!
Since I work only in the adult industry I have experience with working with over 100 affiliate companies directly so should you ever need any other sponsors for other niches drop me a line