Find an image online that you like, or upload one of your own pictures to a image-hosting site like Photobucket or Tinypic.
Once that's done, copy the image url of that picture. It should look something like this:
http://randomexamplepic.jpeg
Then go to your profile, look under
Modify Profile, and click on
Forum Profile Information.
Scroll down until you see the white bar where you can edit your signature. Paste the image url you copied earlier, and then change it so that it looks like this:
[img]http://randomexamplepic.jpeg[/img]
Once you've done that, click
Change Profile, and there it is! You're done. Hope this helped.