Just to be sure, everyone was voting for the picture, not the individual, right?
I would hope no one would play favorites over something so silly as a forum avatar contest where no one actually wins anything, and just votes for what they consider the best and/or most creative images. Considering that no one person seems to dominate the contests here, I don't think it's really been an issue.
Personally, I don't even look at who voted for me before voting myself so as not to skew my choices by paying them back in kind with a vote.