actualy I already made this "mod" about 1.5 weeks ago...now, members cant vote more then ones even if they reset cookies. but still in testing process.
But want to warn u, the way it made right now will make your DB grew pretty fast, becase it inserts for each user and for each image independent entry. So each time someone vote, it creates new entry in the DB. this is best way considering perfomance, also, with this structure will be very easy make stats, to show who voted what images, what was the vote, and who voted for a specific image.