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The email default should still show the image, but the hyper link should not be visable. Now send yourself an email using the email default and click on the link to test it out. If you like the result, cut and paste your test to other email defaults. If you put the marketing message in your "none" email default, the message will go out on all emails that do not specifically have a email default override.
Thats it! Now all your emails contain embedded marketing message. Be sure to refresh you message at least on a monthly basis. While a current message is great, a out of date message on your emails looks lazy and tired.