You can request a banner in GFX Showroom. Then right click the banner, click properties, hilight the URL go to your UserCP, go to Edit Signature and use the image tags [img*]URL of image[*/img], just without the *'s. Or you can just have any pic as long as its size and/or content not against the rules.
Edit: didnt see what Bubba T posted when I posted.
It may seem obvious, but some people dont realise -you cant just use a pic off of your computer by right clicking and going to properties and using that URL. You have to have uploaded or got someone to upload and host the picture online (so that you can get a URL) there are many places to host pictures and im sure some people will host foryou.