> Absurdly wrong, marketers already use a unique image URL for each email recipient, and Google has no way to know that all of those point to the same image.

wrong.

you can of course do simple image processing and identify similar images. if the marketers only change the name(url) of the file and not the content one-bit, you can trivially compare the hash of the file... even if the marketers change content, assuming the marketer sent the image %90 same and 10% customized per person, you can borrow techniques from image compression domain to compress this humongous data very efficiently.