It's oddly hard to get traction sometimes on contests. Is it possible you can take the art or photography winner from someone else's contest and use that as the prompt for your own? (like @gmuxx does for his). That way, you could have the art contest promote YOUR contests, and vice versa. @juliakponsford's art contest is usually full of quality submissions, maybe she would team up with you :)

I think the 7 day window is fine. I've yet to determine what days are crap days for posts and submissions though. Sometimes they all feel like crap days.