I see this has been sitting here with a reply since last week.
Have you checked the web testing forum?
It would seem to me that you've got a daunting task ahead of you. Not just the printer settings and individual printers would need testing, but at least in the case of Yahoo, their ads can be 1/4 to 3/4 of a page within mails. Not sure how you would ensure that an e-mail image doesn't break across pages unless you're able to insert page breaks at the beginning of the image that any browser, mail reader and printer would understand.
"Never doubt that a small group of thoughtful, committed people can change the world.
Indeed, it is the only thing that ever has." Margaret Mead