Laura,
Go to this page
http://www.createarevolution.com/blog/2008/02/07/creating-an-html-signature-in-a pple-mail/. This example worked but the instructions weren't 100% correct. I did get it to work just fine though. You will need a html editor and know how to embed a hyperlink. I use BBEdit for my html editor.
Start by creating a default signature in Mail with any text. This creates the file you will need to replace later. After you have done this completely quit Mail.app. Next you create a html file with your html editor and make your signature to say this "click
here" or whatever you like, with fancy graphics and colors. You will have to code the html correctly to get the colors and graphics right. You will need a HTML editor to do it. Save the file and then put a .html on the end of it. Then you open the html file in Safari by control clicking it and select "open with Safari". Next inside Safari click save as Web Archive, and save it to the /Users/Library/Mail/Signatures/ folder under the same name that is already there. This replaces the current signature with your new html embedded one. Then you open up Mail and send a test message to verify that it worked.
Message was edited by: b3av1s