This sounds a lot like what http://thumb.it/ does. It works pretty well actually and the responses are within a couple minutes
The advantage of having two images is the voter doesn't even need to read a description or know what part of the image they're supposed to up/down vote. They should be able to easily see the difference between the two and just make a snap decision in less than a second. Not having to read text makes it more fun, I would think, and also provides a lot more results for the picture-taker.