If it is possible, try to save the image that you want to use for your avatar on your computer, and then try to upload it, and see if that works. After you save the image, don't use the line where you put the URL for the avatar, use the one right below it. Just click on the gray "browse" button, and look for your file, and then open it. The file's name should appear in that line now. Click the "save changes" button, and see if that works.Also, your avatar can't be any bigger then 75 by 75 pixels or 50.0 KB. Your profile picture can't be any bigger then 100 by 100 pixels or 97.7 KB.
I hope that my instructions weren't too hard to understand!! I am really bad with computers, but that is what I had to do to get my avatar up when I was still using an avatar.