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.
I recently received an .eml file from a client which contained a JPG image but was unable to extract it using Mail on Windows 8.
SOLUTION: Change the file extension from .eml to .mht then open it in Internet Explorer!