You didn't say whether or not you knew how to do this..but just in case you don't....
to get the url from your image that you put on your website...find the properties...it'll be something like
http://www.yourwebsitename.com/banner.gif or
http://www.yourwebsite.com/images/banner.gif.
Also...make sure your website allows direct linking..if not...you'll have a broken link where the banner should be. You didn't say if it was a personal site or just a free website...if it's a free site..test the img src first to make sure. You can come here and put it in your signature for a minute to test it out...using the html code from Pia's post...substituting your own information!
I send all my banner information to the WM in the 2nd form from above....
a href="my referral link"
img src="banner url"
Some email clients will try and take a html code and transform it into the actual banner..instead of showing the code! lol
Good Luck!!