This is long waited basic feature for Gmail. Finally it is there.
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.
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 a different signature for that address. From the Settings page, you can edit the signature for each account by changing the email address that appears in the dropdown menu.
Rich text signatures - Official Gmail Blog
