So there is something I'm not seeing this being discussed anywhere. What about the idea of only storing one image for any and all recipients? Yes I know that the file name is changed based on recipient to try and track the users viewing of the image. But a simple md5sum of the file will indicate that it is potentially the same file as others being cached. Google would only need to store the references and the one file. Thus, the first person to view the file (per maximum cached time-frame) would indicate a viewing of the file, but subsequent users would never be identified as having viewed the file.

I recognize that there are a couple potential downfalls to this thought. 1) The time/processing it takes to determine the md5 could be problematic on such a large scale. 2) I have no idea how easy it is to change an image to be unique for each user.