Just thought I'd mention that personally I wouldn't mind an ad being placed in the return mail. Though I'm sure the privacy advocates would be against it, using OCR to determine keywords and targeting the ads would be beneficial for you as well as the consumer.
Thanks, I appreciate hearing that. I had thought of putting an ad in the email, or even adding one to the bottom of the image itself - though I felt like that would be too intrusive and decided against it (adding to the image itself). Thanks for the feedback.
Unless you could find a solid impression based advertising provider or are willing to spend the time finding advertisers I wouldn't add anything to the image itself except (maybe) the URL to your homepage. For the email bit you'd still have to be careful with the TOS that most pay-per-click ad agencies have regarding email.