The question is similar to one I raised last year. I never did get it working.
Why HTML in the mail body? Well why not? My users certainly prefer html, it looks a lot more professional than plain ascii. Your example is to add an attachment and is easy to get working. The question, as I understand it, is to use an HTML img tag, send the gif with the mail and have outlook display it properly.

Management always feel more comfortable if their corporate logos are displayed.

Update Take out the subtype => 'related', line in the OpenMultipart call and it will work. It just came to me :-)