Actually, as a recipient, I'd be annoyed to have this trigger gmail's "show images?" dialog, only to find upon choosing "display images" that the only thing I get is... a marketing message from the sender.
That said, I still love the idea!
I totally understand that it might be annoying to only get a marketing message from clicking "Display Images". I guess the way I look at it is that it's a chance for companies you're already interacting with to tell you things about themselves that you might not already know - so it might be annoying or if you like the company it might be cool too.
It's also an amazing platform for organizations like schools, charities and political campaigns to do outreach with. For example, a charity can have their big donors install this in their signature and have a unified, centrally controlled donation message going out every day that's always current and updated.
Thanks again for the feedback!
You will wouldn't have analytics, but at least the ad would display.
For example, you can integrate Airtime with GMail but Gmail doesn't actually allow you to play with HTML tags. You have to do it by inserting an image and then linking the image by hand. Effectively it's the same result, but it really limits our options in terms of styling etc.