Thunderbird is Mozilla's mail client. A great alternative to the hideous Outlook or Outlook Express
I use TB, though I don't use signatures so I can't tell you off the top of my head. Download and have a look around
Though I think you should tell your client he will be better off using a plain-text signature. This way all of his recipients aren't always downloading an image file (some people like to view emails offline, for example. Or those with slow connections), and to place the image inline as a hyper link, he will have to send all of his messages as HTML. Some people do not accept HTML emails.