I don't think many players would agree it as fair, plus, we won't get to see the nice screenshots created by others except the winner. A good idea is what we have running at the SAS forum.
http://www.sas1946.com/main/index.ph...c,19375.0.html
http://www.sas1946.com/main/index.ph...c,18631.0.html
Where each user posts his pic, then after the thread is locked, voting begins by all users and the winner has his screenshot featured on the next comps initial post by the moderator. Usually providing a caption or title to each post and then that is used in the voting poll along with the users name.
Each one has a theme which must be followed, or the post removed.
We would probably need a new moderator for this specifically.