You can't upload your avatar directly to this forum. It has to be linked from some image sharing site such as Photobucket, Imageshack, or DeviantArt.
Simply click one of many large upload buttons there after making an account, and then copy and paste the direct link into a little box at the bottom of your options tabs in your User CP area and make sure to click the custom avatar bubble.
Which means, as long as you have internet connection on either, you can upload it to the forum.