Go to the User CP link at the top of the page, click on "edit signatures", and click on the picture icon above the text box. A window will open asking you to paste the link to the picture.
you'll have to first upload it to an online source. I use http://www.photobucket.com. You sign up and upload pictures from your computer to the internet. Photobucket will generate links to those pictures and you can use those links in your signature, as long as they follow the guidelines in the first link I gave you.