Hi Michele,
I agree that only being able to pick a single favorite is better because it often forces us to look at the images more thoroughly in order to make a decision. On the other hand, when I only allowed a single vote, there were some very, very good images that weren't getting any votes because they were competing against an exceptional image that week, and that didn't seem like it would be very encouraging.
I'll have to see if I can think of a better option.
Keith