sfrr: How about this:
*You run the first competition, and choose a theme.
*Members make their entries.
*You pick your 10 favourites and put them in a 7-day poll.
*The members vote their favourite.
*After 7 days the winners are announced.
*The overall winner gets to choose a new theme.
To keep it all official we should have the winner PM you their chosen theme so that you can run each event yourself.