Go to your forum profile, and in the 'Signature' box type:
[img.]http://-insert the url here-[/img]
and tada you have the image (also, add the [center.][/center] code as well in order to have the image in the middle of your signature space). Another thing you could do is write it in a post on any thread here (obviously not posting it) and copy the coding of it on the forum post and paste it in your signature space.
(Remove the . from the img and center code in order for it to work properly)