Press the user cp button

then click
Edit Options scroll down this page and press change avatar button.
On the next page you will see
You can upload an avatar from your computer: and then a Browse button, press it to upload your avatar from your HD.
Note: the maximum size of your custom image is 64 by 64 pixels or 10000 bytes (whichever is smaller).
To get help choosing the right avatar go here:
http://www.techimo.com/forum/t9964.html
There are a lot of gifted artists who will be glad to help you out.
Regards