If you have an URL for your favorite avatar, you can add it in!
But please try to keep your avatars at a size of 80 x 80 pixels, as a courtesy to the other forum chatters.
Avatars larger than that run the risk of being deleted without warning.
Avatars with inappropriate content, slander, etc also run the risk of being deleted without warning.
To add an avatar simply go to your profile
profile.php?mode=editprofile
And scroll all the way to the bottom, and input the URL to the avatar of your choice.
You will need the URL address to the avatar, and if you make your own avatars, you will need to upload the avatar to your own webspace.
You can try http://www.myimgs.com/ as a place to host your avatars.