Shouldn't be allowed
Under normal circumstances, I would tend to agree with you.
But in this case, we're usually talking about 4 pics which for all intents and purposes are equal in their overall visual appeal. So essentially, it's a matter of splitting hairs.
As I have been the most notorious for flipping votes in the past, I must admit my "flipping" has sometimes been a strategic move in order to help solidify 2nd place for a particular candidate.
