You can now use rich text as well as images in your GMail signatures.
Currently GMail supports hotlinks to images for using images in your signatures. You can use Photobucket or Imageshack to use inline images in your GMail signatures.
Goto Settings, scroll down on the General tab for Signature section to modify your signature. Also use Insert Image on the Signature textbox toolbar to link images. Make sure you upload your images to above mentioned sites or any image hosting provider that supports hotlinking
