To have an anonymous contest you would need a place where people could post an image privately then have the image assigned a number and posted to a public gallery. Someone would have to keep track of whose name goes with what picture. If I were to post my image in a thread you would see my screen name. You would also need to be sure that none of the images had signatures. This is impossible for an unofficial contest. It would require a moderator with public and private access to a web space to do the functions required for anonymous posting and voting.