Note: These instructions are for the Reno 911! fan forum. However, it depends on what forum you use these on.
First right click on the avatar. Then click "save as..." Then save it as whatever you want. Now you need an image hoster to link it to, I use Photobucket.com It's easy to sign up with. At photobucket, simply click the upload button (once you're all signed up, it's free for now) and upload whatever picture. When you've done that, go to the fan forum. Click profile. Scroll down to "I have my own pic" and enter the avatar's URL (the link to it) BELOW IT YOU MUST PUT THE SIZE! I HAVE THE SIZES LABELED BELOW. IF YOU DO NOT THEN IT WILL AUTOMATICALLY RESIZE YOUR AVATAR AND MAKE IT LOOK WEIRD! Have fun :-)