click on 'my page' at the top of this page, then click on 'edit options' and scroll down to the bottom, hit the 'change avatar' button.
Now in the 'use custom avatar section' check the 'yes' box. If your avatar was saved to your computer already then hit the choose button on the last line and find the avatar on your computer and select the file. Then hit 'submit modifications' button and it should work.
If it doesn't it could be too big (bigger than 50 by 50 pixels) or it means the title of the file may contain spaces or punctuation-I fought for a week with one of my avatars to find out I had a '&' in the title and it wouldn't accept it.
Hope this helps. I'm assuming you already know all this stuff I wrote and it's some horribly complicated problem and I know nothing but I tried
