When this happens, it happens subliminally. That is, I don't think anyone is deliberately not down-voting others who have a photograph of a friendly, smiling human being as their avatar. But that doesn't mean the effect is not there.
You'd have to actually examine voting patterns on a per-user basis, and for that you'd need access to the (actual) DB. Just asking people without access to the real data won't get you real answers, you'll just get everyone saying "No, that doesn't influence me", because they are not aware of the influence.
Well, except those who claim to freely downvote uggos. But that's another matter entirely.
This is similar to the way that people claim they aren't influenced by advertisements, when in fact they almost certainly are (ask Alex Papadimoulis!).