It only works in the latest desktop version and it also supports a unique signature for each email address associated with your account.

http://gmailblog.blogspot.com/2010/07/rich-text-signatures.html

<< Rich text signatures have long been one of our most widely requested features. Some of you have tried your own solutions, including Greasemonkey scripts, browser plugins, and even using canned responses from Gmail Labs. Others have simply lived with frustration of not being able to change the colors or font size of your signature, or insert images and links. Either way, you'll be happy to know that today we're launching the ability to write your own rich text signatures right in Gmail...

The next time you log in and visit the Settings page, you'll see a rich text editor in the signature section. Here, you can customize your signature by adding pretty formatting, links, and images — or decide to leave things nice and simple. >>

http://4.bp.blogspot.com/_JE4qNpFW6Yk/TDYhaX9aJCI/AAAAAAAAAok/SUZMdB9N7sI/rich_text_signatures1.png