Seems pretty trivial for marketers to work around by making each image request a unique URL per-recipient. Assuming Google's proxy fetches the image when the user opens the email, you could use that technique to find out when a user reads the sent mail.
You wouldn't get the IP address like you would with conventional bugging, but you could still find out how many users read the mail and what time they did so.
I thought that at first too, but to deduplicate, they still need to first issue a request for each, download, and compare. So the requests have already been made, and the marketer received their information.
Dedup on image data has no bearing on the information advertisers can collect. On image URLs it will cause advertisers to append query strings to image paths. Google can't win that particular arms race, but grabbing the images on email receipt instead of on email viewing neutralises any gains from that win.
One point nobody has raised yet, though, is that there could be valid use-cases for the sneaky stuff people were doing before. Images generated on the server that reflect current (updated or updating) info could be handy. It might even be worth serving different images to different clients based on user-agent strings. I'm skeptical on both counts, though.
That is only possible if the URL is identical. If senders use unique URLs per recipient, Google could guess at which images are identical based on surrounding e-mail content, patterns in URL components, and/or statistical sampling of some of the URLs, but a 100% accurate deduplication would either require identical URLs or a lot of bandwidth to read 100% of URLs.
Well Google already crawls a ton of URLs so it's hard to imagine bandwidth being a major issue, right?
Even the same URL can return a different image. That wouldn't be super useful for tracking, but they can only truly dedupe if they read every response.
You wouldn't get the IP address like you would with conventional bugging, but you could still find out how many users read the mail and what time they did so.