If you are on the mac, and use mail, you can set up a signature via Mail>Preferences>Signature, but not an image, per se.

However, a great workaround I have found that allows me to insert my business card image, a logo, a small ad, a note about an award or whatever, customized to every email on the fly, is to use Typinator (http://www.ergonis.com/). Again it is cool because not only can you, with one little keystroke like ‘addlogo’ or even ‘-al’, you can insert at the moment you need it any image, OR any text. great for boilerplate responses, creative briefs and etc.