If you're looking to jazz up your Mail signature, try this:

1. Ensure you have one signature setup in Mail
2. Quit Mail
3. Create a web page in your favorite editor
4. Open the page in Safari
5. Save the page as a web archive
6. Place the web archive in the ~/Library -> Mail -> Signatures directory, renaming it to match the signature you want to replace.

Note: You may have to open the plist file to determine which webarchive maps to your signature name, based on the number of signatures. Have fun!