Comment by mjbraun

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.