I wouldn't necessarily say 'judge' as that is a bit of a strong word,
usually I will make assumptions about the person, but I will not act upon them until I have formed my own opinion through our interactions. For me it isn't just about the avatar though, it's about the whole package, whether they put thought into their posts, the signature, what kind of stuff is in it, the profile, I'll make all kinds of assumptions and I'm not afraid to say it, i'm human. I'll assume things about them from their age, sex, how they write, what they write, everything that a person does.
It's just what I do, my assumptions get proved wrong sometimes, and I can deal with that.