If someone doesn't want to take the time to post decent pictures of their car to help it get sold then just don't bother with trying to buy it. I'd rather let the "market" deal with it then force some arbitrary image dimensions on everyone.

Who's to say that even if we do that the pictures will even be useful? Will a grainy camera phone picture be acceptable even if it meets whatever dimensions we pick? I don't think forcing everyone to have a photo shoot of their car is the best solution to the problem.