Thought about this, but if we open it up to users some of them may upload photos that are not about food, right? Or maybe we can do some moderation before including them, or even allow users to weed out non-food photos.

There's your problem right there. If I can't upload my photos and compare them to others and be judged, then I have no use for the service.

There are solutions to this problem, but censuring, controlling or impeding user contributions is not one of them.