Find the avatar you want, and download (save) it to your computer. You can Google "avatars" and see thousands of them, or be more specific in your search. You can also use Google Images to search for them. It will need to be 80x80 pixels or you will need to reduce it to that size.
When you have the image you want, in the proper size, saved on your computer, click on your user name (in blue), then click on "User CP" on the blue menu bar. In the left column you will see an item called Avatar--click that and then upload your image.
It's really easier than it sounds.