Awesome Idea.
A few pointers:
Have the competition in the Screenshots board
Limit entries to one screen per member
Provide parameters for the size and dimentions of the picture, or you will be overwhelmed with 4MB posters.
Have a set deadline for entries
Pick a theme per competition, and have regular competitions.
In Modact 3 there is an option to take screenshots in either jpg, or tga. TGA is much higher quality. Members can extrapolate these mods to their UP3 or HSFX installs also. DBW which will be out later will have it already.
Let me know if youre interested in being the game-master for such competitions, and we'll give you a hand.
cheers
CirX