but I'd be wary about how that email will render between the different email clients and where you ultimately host the binary image file
It's just an image swap, so implicitly has the same dimensions.
I was simply describing what services like Moveable Ink do, while being product-agnostic (since you could build the same functionality yourself using any image-generation library, it's not as awe-inspiring as they try to imply!).
Ha, Sanford Whiteman! I'm in awe of a lot of the things that you come up with on here! I do get your point about a third party app being able to do this though... Do you think that there'd be a risk of increased issues from spam/phishing filters with an image on a Marketo email coming from a different non-Marketo IP address though? Just spit-balling here, but perhaps you could adjust your SPF record to help mitigate that as an issue...
If that's not an issue at all - and the business need mandates dynamic content at the point when an email is opened - that sounds like a really good solution... at least from the HTML side of things.
No worries about it increasing the spam score, but there's a spam-related factor: mail clients are under no obligation to re-fetch the images in an email after first caching (and scanning) them. So the underlying expectation that an image will be refreshed over time is flawed; even if the end user hasn't seen the image yet, they may see the original image, not the "current" one, due to caching.
With any tech like this, you have to look for-slash-intuit the fine print: what portion of your leads are going to have the desired experience, and how bad will it be for those who get the fallback experience? I turn images off except for whitelisted senders, and my app only downloads images once. So I might well see the first image in the sequence, but it'd better be workable on its own.