I’ve been using the Mail app on the mac often these days, and I wanted a way to imbed images for a custom signature I had in mind, but adding a signature through the normal way does not allow much functionality other than the normal font, color, and some minor text effects.
Luckly, through some digging I was able to find this blog. Follow the necessary steps and you can have images in your signature as well. Along with any CSS you want to add.
http://allforces.com/2006/04/14/css-signatures/
Enjoy!