Gmail: Rich text signatures
Rich text signatures have long been one of the most widely requested features. Some have tried their 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 their signature, or insert images and links. However,you now have 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.