forgot to add a feedback on the challenge itself. Its an awesome idea in general, some creative thinking is definitely a great way to relax after stressful work day, plus it gets your ass of the couch to actually go do something outside your home ;P I think its only fair if pictures running for the prize are taken for the contest specifically, or at least within a certain time frame. Lets say you announce the theme on Monday and give everyone a week, that means all contestant must take the shot during this week and don't use your previous works. I feel like that gives a fair start to us all, pros and nonpros.
Also have this though, Idk if that will work, but here is the idea : what if the winner of current challenge will be judging next one or help judge or pick the theme :P