This gets very interesting when Bills shows you how to add a custom image to your signature. You actually have to encode the image directly so it arrives with your message. Otherwise, spam blockers may block the image download if you just use an HTML link.
It's a really easy to follow method and I had very little trouble getting it set up to include the signature shown here. My biggest obstacle came from my tendency to paste everything in TextEdit. For this project, PropertyListEditor works better and more reliably due to the length of the pasted key.