Copy and paste the following into your signature:
[ url=http://www.fm-base.co.uk/forum/f81/conquering-the-continents-t41799.html][ IMG]http://img190.imageshack.us/img190/5901/storysig2.jpg[ /IMG][ /url]
Remove the spaces in the url and img tags.
Then when people click the picture, it'll take them directly to your thread, so you can remove the existing link in your sig.