The first thing I'd do is check the output - see if you're getting valid HTML that includes the images you want to see before this gets attached to or included in your mail. If this looks right then I'm inclined to think that the images are being stripped somewhere along the line by your mail server, or something external to SSIS in any case.

To boil it down, first make sure whether or not the issue is due to what you're doing in SSIS.