They are already scanning the content of the emails so there's nothing stopping them from determining if the images between emails are the same regardless of name, and even then just sending down whatever they have cached, 1px transparent gifs are still common for this. Could break some a/b testing software though. You could see marketers move to including a unique image per recipient like a gravatar. If it were me I'd just include something like the github avatars in the footer of every message. Google can learn that those are tracking images and block them but will they?

"Thanks, the next question is if gmail sees a bunch of emails from the same sender with these hash-named images, I wonder whether they will squash them."

Are you saying squash the sender or squash the tracking images?

I hope gmail doesn't start squashing my emails because it contains a tracking pixel.