And you can see where this leads... each week you'd have one Q&A thread for that week's competition... and everyone could vote directly on the comments containing the photos. It would be obvious who the winner is (the most upvoted comment), but for posterity that would be recorded by selecting the comment as the best answer.
I think you would also need sub-comments for each answer in the same style as stack exchange as otherwise people's comments on the submissions would get re-ordered away from the subject and the thread would read a bit odd.
And you can see where this leads... each week you'd have one Q&A thread for that week's competition... and everyone could vote directly on the comments containing the photos. It would be obvious who the winner is (the most upvoted comment), but for posterity that would be recorded by selecting the comment as the best answer.