Go to "Profile" at the top of this page. Once you're there, below the second blue bar, "Profile Information," there is a "Signature" field. Put the following there (replacing of course "someimageurl" with your picture's address):
Code:
[img]http://someimageurl[/img]
That will include your image after your posts. The "avatar" field further down refers to the picture on the left side only.