How can I create an HTML signature

Go to the Signatures preferences pane and edit one of your signatures. Here you can add the raw HTML needed for your HTML signature. When using such a signature MailMate generate both a plain text and an HTML variant of the email. Therefore you still need to define a plain text signature for the plain text part.

Images are best done by using external links (using a href). In other words, you need the images to be located on a server somewhere.

I hope this helps. There's also a little bit on the subject in the manual.