I can agree with previous posters that it varies or the choices are random. For some I guess there is something deep and meaningful behind their choice, for some it's just more spontaneous, in the heat of the moment thing even. I guess some use pictures of how they would want to be perceived in real life as well.
You can see what the avatars "tell" to others in that avatar psychoanalysis thread. Sometimes they're interestingly spot on, sometimes way off. Usually I notice other people's avatars when they immediately catch my attention. I don't psychoanalyze them at all. But I've also spent a very long time reading somebody's posts and I do recognize their avatar, but all of a sudden I realize that there's something/somebody on their avatar that I didn't notice before, so the avatar is not what I thought it was after all. It has happened to me several times.
My choice of avatars is usually based on feeling, the picture somehow seems to capture something I'm currently feeling, it might not even be visible on the picture, it's just the vibe I get from it. I change it when it doesn't correlate with the feeling anymore. I don't have an avatar at the moment because I haven't come across with a picture that would "feel" right. Usually I choose pictures of women, I've tried but it feels weird to have something inanimate or a male form on the avatar. I've also more often than not chosen faces that are partially hidden, it feels weird to have somebody else's full face "representing" me, it feels wrong somehow. And there's no way in hell I would put my own picture up as my avatar, even my own face doesn't feel like mine.