You upload the avatar to the server. I found mine on the net by doing a search on google for "clock spinning hands" and found what I wanted in under 30 minutes.
Click on "user cp" in the tool bar up top, then "edit options" and go to the bottom to "change avatar". If you have something on your computer that you want to use, the rest is easy.
On the next page, where the standard avatars are shown, go to the bottom, and click in the small circle labeled "yes" next to "use custom avatar" and click "browse" to find the file on your computer.