Avatars are the little pictures under your name when you post.
You can select what pic you want by clicking on your screen name in the upper right corner and selecting "My Profile".
Then click on the word "change" that is just to the left of the generic photo you have under your profile description.
Unfurtuntely with this version of the board, we don't have any pre-selectable options to pick from. Instead I will upload some options in this thread. Once you find the one you want to use for your own profile, right click the photo and save onto your desktop. Then follow the instructions above to get to your "My Profile" area, and click the "browse" button to upload the avatar you just saved on your desktop.
Kinda sucks, but whatcha-gunna-do.