Well, depending on how many folks wind up competing in this contest, you could break it up into something like so:
Round 1:
Challenges!
- a forum member is allowed to challenge one or two others to a 'duel' of artwork. Those members may either accept or decline the offer. Any who accept will then be pitted against the other artist and both given a prompt to work with, like: "Irate Velociraptor", and will both be given a certain amount of time to work on their picture. Pictures can be in any media.
And then you work from here. I get this idea from a competition called "ThunderDome", which is a competition between many artists, professional and otherwise. Great stuff in those contests.
Just a suggestion
Ang
"A foolish consistency is the hobgoblin of little minds, adored by little statesmen, philosophers, and divines."
-Ralph Waldo Emerson