Tracking pixels and tracking links only work because there are unique identifiers in the URL. So if you just reference the image's direct link in the HTML of the email there's really no information to be gleaned outside of the normal email server handshake.
However, when Google proxies the image in an email, there is no way for the user to know the original URL and see if it has a unique identifier or not.
However, when Google proxies the image in an email, there is no way for the user to know the original URL and see if it has a unique identifier or not.