Sensible answer 1: Have you checked whether you are sending the message as Rich Text or Plain Text? Also the image will be included as an attachment. If the remote email program can't display it inline that's it's problem not Mail's.
Rant Answer 1: Including an image in every email just because it looks nice is an incredibly inconsiderate thing to do. Think about all of the extra bandwidth and server storage that goes on so that you can send an attachment containing your picture.
Sensible Answer 2: Instead of including the picture in the email host the picture somewhere (
http://some.hosting.net/mysig.jpg) and just provide the HTML tags in the signature to refer to the image ( <IMG SRC="http://some.hosting.net/mysig.jpg">). This would solve your problem and make you a better internet person
Rant Answer 2: HTML is for web pages not email. Many of the problems with viruses on Windows are causing by the processing of HTML emails. Now that Mail uses WebCore in 10.3 there is the possibility of such viruses making their way on OS X. HTML email, Just Say No!
