Maybe a monthly theme? It could involve the holidays for that month, the time of the year, spring, summer, fall and winter or types of photography, fine art, portrait, landscape but all involving nudes.
The prize could be a free premium month of newbienudes and a stamp on your profile.
You would be limited in your entries, maybe just two or one for that month plus you would have to be a verified member.
These are just my suggestions on the matter.
Thanks!