An avatar is generally any jpg file that you want. Once you find one on the internet or pictures of yourself, you should resize it to make sure it's not too big.
The maximum size of your custom image is 200 by 200 pixels or 40000 bytes (whichever is smaller).
When you're file is ready click on the "User Cp" button at the top of the screen, then click on "Edit Options". Now scroll to the bottom of the page and click on the "Change Avatar" button. On the next page you can put in a link to your avatar or just upload a copy of the file from your own pc.
Hope this helps.