*looks at question.*
Let's say you have a cool site whose address is:
http://www.mycoolsite.com/
Let's say you have an image in your signature whose address is:
http://www.mycoolsite.com/mycoolpic.jpg
Now, let's say that you desire that when a person clicks on this image, he/she will be taken to your cool site.
Then this is how you will code it in your signature (without spaces in the tags):
[ URL=http://www.mycoolsite.com/][ IMG]http://www.mycoolsite.com/mycoolpic.jpg[ /IMG][ /URL]
Essentially, you enclose the image code with the url code.
How's that?