With Google pre-fetching all of these, every GMAIL address id Verified for the Spammers.
But Google doesn't need to prefetch all of them for everybody. That would be merely equivalent to just flat out *removing* their "Load Images" link feature. Maybe that is what they're doing, if their marketing department has gotten vicious enough. But they may be smarter than that.
Presumably, a spammer will send the same image to a million email addresses using a unique image URL for each one. For this maneuver, all Google needs to do is load some images that arrive in emails that bounce or that are addressed to a population of dummy recipients. They can store them along with the results of any algorithm that can recognize the same image when someone else gets it from a slightly similar URL. If a few thousand dummy recipients are getting the same image from unique image URLs, they can test whether the images look the same and how they can doctor the URL in various ways without changing or losing the image. If they figure it out, they can perform the same munge algorithm on the image URL in your email, or simply display the image they have already cached from their dummy recipients without hitting the external image server at all anymore.
Your example of devious spammers using single-pixel images is a really poor one. Google probably has all those images stored somewhere already.
Its not a well thought out scheme at all. No sensible person would read Gmail with a web browser from now on. The wise choice is to use a traditional Email Client, (something like Thunderbird, Kmail, k-9 mail, Evolution, etc), and set them not to load images at all.
Or, go to Settings, click on the "Always ask before displaying external images" radio button, and then on "Save Changes".
There are plenty of reasons to avoid Google, but this one isn't very compelling.