I'm curious about this too. I know that I've occasionally gone back to an image that I'd forgotten I'd already voted on and hitting the vote button got an error message that I'd already voted. But I know that I've voted for more than one image in a day and didn't get any message, so I assume that you can vote more than once. I'd like to know, because after voting for something, I've come across something that I really felt merited a vote and didn't vote cos I didn't know if I could. If anyone knows for sure, I'd like to know.