Gmail now supports to add images in signatures. For that navigate to the Settings > General > Signature. From here you can edit the signature for your account. Currently, only the latest desktop version of Gmail supports rich text signatures and multiple signatures.
Now you can only include pictures and icons in the signature that are already hosted on the web. And you can not upload images directly from the computer straight into the signature. You can upload your pictures from your computer to Flickr or Picasa or Photobucket and then give its URL to add signature.
