[aezell]

Only for repeat opens. The original open still accesses a particular URL specific to that recipient so the open is recorded even if Gmail hides IP, browser, OS, and other data via their proxy. If the user doesn't view the images, that open is never recorded anyway. So, the caching doesn't actually affect the open rate.

[glomph]

They cache them before the email is even read. They also cache images across gmail accounts.