HTML email signatures are one those things that ever self respecting business should have. But they can be a little tricky to do properly. Here's an article on The 17 Email Signature Do's and Don'ts which covers most aspects.
One very important thing that wasn't mentioned was "Don't place important details inside images", such as your name, phone number, website, etc. The reason for this is that if the person receiving your email has images turned off or can only receive plain text then they won't receive those details.