To be in the spirit of things, I uploaded one of my facebook photos as my avatar. OK, well it's me, and my much more beautiful wife and kids. But hey, no one wants to look at my bald, bearded head anyway.
I used to be very conscious of avoiding linkage between my internet aliases and my real identity. I came to realize that the government and megacorps like google or my ISP can easily figure out who I am anyway by linking emails to internet purchases. I gave up on it. That doesn't mean I don't use email addresses and passwords different from forum logins (to thwart hacking attempts) but I gave up on real privacy...I go with a semblance of it, instead.
That said, I normally prefer an avatar of something I really enjoy as an avatar. My previous avatar was a miniature giant space hamster from the classic RPG Baldur's Gate II, with an eyepatch someone gave him to make him look like a pirate. Because I love Baldur's Gate
Sometimes I also go with something that stands for something...I was using this anti-DRM one for a while...
Sidenote:
One of the forums I used to frequent a lot, hometheaterforum, had a policy that you had to use part of your real name for your nickname. They said it helped to maintain decorum - people are less likely to be ***holes to each other when they are using real names over screen names. I dunno if it really did any good or not.