Of course if you are really insistant on this I believe you just send emails like normal execpt you use HTML code markup in the email, and need to change a setting or two so that the email program you are working on recognizes that the outgoing email is HTML.
Ok what email client are you using? From what I read it seems that Mail.app doesn't have any options to send HTML emails. Though it is possible with that apple script posted earlier or perhaps a plugin of some sorts. (See you need to be able to change what type of email you are sending from text to html, which you can't really do in Mail.app).
You might want to try another client like Thunderbird. I read that there is an option to send email as HTML. Then all you would have to do is writeup a little html code to display your banner or whatever and then send it.
Of course the images would have to be stored on a server somewhere since sending the images as attachments won't work since most people won't bother opening the attacment.