How can I use HTML/CSS to add a background image to an email?

Question

I've been trying to get background images working in my Email HTML and have had no luck. I'm using Outlook 2007 to test my HTML code.

My method is:

• Create my .htm file
• Save it in %appdata%\Microsoft\Signatures
• Create a New email and set that HTM file as my signature

So far, all my attempts at adding a background image have failed and I'm beginning to think it's not possible. Any ideas?