You firstly need to host the image somewhere on the internet (try geocities.com or search for other free hosts on google.com).
Once you have uploaded your image on a website, and have it's URL [universal resource locator = path to the image], go to UserCP on the forums and then to Signature, and type in:
[IMG*]http://www.whatever.com/whatevah/whatever.jpg[/IMG*]
(with the exception of the *).
There you go, if you do this - you have an image in your signature.