... You may want to set a "not older than..." limit to avoid someone posting an image that was produced in May 2008 to the June 2011 contest, for example. If I was to make the decision on this I'd set a "seasonal" limit, something like the past 3-4 months or so, but if you'd rather not to impose a limit, I suppose that's fine too... Just a thought...
BTW I was surprised to see the contest idea... It's interesting to say the least!