"Rich text signatures" have long been one of the most widely requested features. Google have now "rolled out ability to write your own rich text signatures right in Gmail," –– without requiring any hacks like Greasemonkey scripts, browser plugins etc.
"The next time you log in and visit "Settings page," you'll see a rich text editor in signature section. Here, you can customize your signature by adding formatting, links, and images."
Gmail also now supports a "unique signature" for each email address associated with your account. So, if you send mail using a custom "From:" address, you can use different signature for that address. From Settings page, you can edit signature for each account by changing email address that appears in dropdown menu," explains Google.
Unfortunately, this feature only works in latest desktop Gmail interface.