i just did the same. the image is a link in this way. so if the recipient wants to re-read his e-mail off line, he will get the broken image links. well, people have to live with this broken image links thing. i tried to find the solution for that in the past few days, such as embedding the images into e-mail body. no easy solutions i found so far.
and the images should be in full http address. or you should have a <base> tag in your html file.
well, if you find better solutions, let me know.
hui