WARNING: YOUR SIG IMAGE MUST BE NO LARGER THAN 100x100 PIXELS IN SIZE!
Failure to adhere to this SIMPLE rule will result in an administrator gleefully deleting your entire profile. Why? Because using larger images can screw up the layout of the message board plus they eat up bandwidth.
To add the image, you first need to have a 100x100 (or smaller) image available somewhere on the Internet. For instance, let's say you have an image at: http://www.myhome.com/mypage/myimage.gif.
Take that url and enclose it in an image tag, like so: