After you have the URL for the picture you want to use (try to keep it about the same size as the rest of ours if you're making your own, this way it doesn't screw up formatting for everyone)..
Then go to - User Control Pannel (in the bar on the left side, right under posting rules) - Profile - Edit Signature - Post URL with the tags about it.
Mine for example looks like this:
[code][img]http://i41.tinypic.com/bfjn8l.jpg[/code]
If you want to upload a picture to get a URL there's several sites to use for free. I use tinypic, but there are plenty of others.