First you need a picture no bigger than 64 x 64 pixels. If you don't have one, there are websites where you can download them ready-done - there is an extensive selection at in various categories at:
http://dreamquest.to/AvatarVault.htmOnce you have the avatar of your choice, click "usercp" at the top of the forums, then "Edit Options". Scroll down to "Avatar" and click on "Change Avatar". At the next screen (after it thanks you for updating your profile
) click "Browse" to find your picture, then click Submit Modifications.
If all goes well and the file is not too big (best save in jpg or gif format) your Avatar will then appear next to all of your messages.