Step one:
Have the avatar that you like hosted somewhere on the internet. Someplace like the free photo albums here on redtailboa.net.
Step two:
Open the pic in a web browser so you get the URL in the address bar. It will read something like
http://www.anyserver.com/myavatar.jpg. Highlight the address and copy it to your clipboard (either edit/copy or CTRL+C).
Step three:
Open your RTB profile by clicking "Profile" in the options at the top of any given page of the forums here. Paste the URL into the "link to an offsite avatar" line in your profile (either edit/paste or CTRL+V)
Step Four:
Click Submit and check that it works.
Good luck!
-Your Sick Uncle Morti.