You should type something like this:
[url=http://renaissance-mmorpg.com/][img]http://www.renaissance-mmorpg.com/siggen/_avatar_created.php?banner=08&name=The+Example&font=01&shadow=y&size=10&color_r=255&color_g=255&color_b=255&HexInput=[/img][/url]
Then you'd get this (try clicking it =D):

Now, this all probably looks like hocus pocus to you, but I'm going to explain it all now =D.
First, you've got
[url=
this is the beginning, and makes sure you can click on the signature to go to the site, after that, you have to place the link to the site you want, so for example, you want it to go to google when you click on it? then you type
[url=http://www.google.com
Now then, to close off this part of the code, you place a "]" so it looks like this then
[url=http://www.google.com]
Now comes the image, or text you want, if you want to place an image, you first have to type
[img]
behind
[url=http://www.google.com]
so it becomes
[url=http://www.google.com][img]
After that, you paste, or type the location of the image (make sure ONLY the image is shown on the location of the image, for an example:
This is good:
http://www.renaissance-mmorpg.com/siggen/_avatar_created.php?banner=08&name=The+Example&font=01&shadow=y&size=10&color_r=255&color_g=255&color_b=255&HexInput=This is wrong:
http://www.google.nl/ )
Now then, for our google link, we'll use google's image =D (to be continiued, ran out of time, sorry =()
The rest of the 'tutorial' (edit button dissapeared >.<)
http://www.logoogle.com/images/logooward/january%2005/hot-red-google-logo.gif This link should suffice =D
After you've pasted that link behind your code, it should look like this:
[url=http://www.google.com][img]http://www.logoogle.com/images/logooward/january%2005/hot-red-google-logo.gif
Now then, as a finishing touch, you'll just need to place
[/img][/url]
behind it, the / means that it is the end of a function, the img notes at the link being an image, and the url means that there's a hyperlink, so the end result should look like this:
[url=http://www.google.com][img]http://www.logoogle.com/images/logooward/january%2005/hot-red-google-logo.gif[/img][/url]
And how does this look like on the forum? Like this:

(and yes, you can click it =D)
Did that help?