Simply click on your the User CP button up at the top of the page. Then go to edit signature. Make sure you have the signature uploaded to an imaging website because you'll need a link to the picture. Once you do all that, simply copy the direct link to the image and wrap image tags ([ img ]direct link here[ /img ]) around it in the signature text box.
Er, hope that helps.
>: get out Crimson Crashing.