OK. I'll assume you don't know how to post images to the site in general then. I'll aslo assume you want to use your own photos, and not a photo that you found on Google.
This code [*img]http://www.trainboard.com/railimages/data/500/Trains_038.jpg[/img*] produces this image when the * are taken out of the IMG tags. If I went into my "Edit Profile" page and pasted the above code into the "Signature" box, the pic would show up in my signature instead.
-Dan
Builder of Bowser steam! Railimages Site