will work just fine. So, Google will follow the link and compare the files and... wait a second, the information we care about - that slg read my email - has already been transmitted.
They are unlikely to do that though: it would waste a lot of bandwidth requesting images that users are never doing to see because they'll delete the mail before opening it. Google may have bandwidth and server resources to cobble dogs with, but they are not going to waste it like that I assume. Also if they did you could easily perform a DoS attack (or just give someone a big bandwidth bill) by sending out a pile of email with an image tag pointing to a large object on a competitor's web servers.
An image link in the email to:
http://thisalways404s.com/404/slg_read_my_email
will work just fine. So, Google will follow the link and compare the files and... wait a second, the information we care about - that slg read my email - has already been transmitted.