However, if they make requests (that they don't necessarily have to keep) for images for all accounts (preloading on receive and not read), it does the opposite, which is a good thing.
The problem is, if the filename/URL is unique to the user like "spammersite.tld/images/50093825343.jpg" and 50093825343" is tied to my unique email, then on Gmail's download and caching of the image, they've validated my email. If another email has 023503850485.jpg, gmail wouldn't know that the underlying file is the same unless it loads it. I don't even have to have checked my mail for this to happen.
