it would be hard to police against cheating, as most cracked games came with a trainer. Even with UAE you can use UAE to cheat in some, if not most games.
I'd rather say that each game should be voted forward throughout the week, say sunday to wednesday is nomination day, then you (since you brought the idea forward) collect the nominations and run a poll from wednesday through friday. Then the weekend is screenshot time.
We'd have to let the consciousness of members keep them from cheating, unless you demand them to come play it at your rig at your house, there's little you can do to be sure, except make a lot of rules that makes it harder to participate.
You could however ask that with each entry, people should tell if they played on real hardware or on UAE. On some games, even NTSC/PAL settings could give slight differences.