I love the Calendar idea, and I'd like to add my two cents as far as some of the rules. This is just my opinion though, thought maybe it might atleast strike up some interesting concepts for rules.
It might be said that while one person's images may be exceptional enough to use on every page of the calendar, there may be other people who have maybe 1 or 2 drop-dead gorgeous and/or creative images that deserve to be in the calendar to. A pack of seeds to the person with the most images picked out to be in the calendar. A free calendar or if rollitup can't afford free then maybe half off or something to runner-ups would be cool.
What do you guys think?