This summary is garbage and complete misrepresents the implications of Gmail's change. (I already researched this last week and developed a solution to avoid cacheing with in-progress email images that might get replaced with final versions)
Every singe email marketing system already uses a unique image URL to identify a given recipient. This is frequently called a "tracking pixel" because it's usually a 1px transparent gif stuck in the corner of an email where it won't be distracting. In fact, this method has been used for web tracking as well for many years. It's how Google Analytics originally worked.
Since these unique images will still get loaded when an email is opened in Gmail, marketers will still be able to track your opens. What they won't see, however, is how many times you re-opened the email. And since the image gets cached and requested through Gmail's proxy, marketers won't get information about your machine like browser, IP address, etc. But if you click-through on a link, or you visited their site before (highly likely if you're on their mailing list) then they have most of that info anyways.
This caching by Gmail is primarily to speed up Gmail since it means images can be loaded and shared on Google's Content Delivery Network which is almost certainly faster than servers owned by the email campaign provider for image hosting.