[signal11]

> I've always thought that images within emails would also serve as a read receipt to the server that sent them when enabled or shown

Yes, that's exactly what happens. Proxying the image only hides the user's IP address. If the images in the email load from external resources like https://example.com/fetch-resource?id=something_unique GMail has no way of knowing if something_unique uniquely identifies a user.

This is why disabling images is helpful. Of course Gmail also lets you enable images per sender, and you may find that quite acceptable for sites you have a relationship with (e.g. a shopping site which already knows your IP address and is sending you delivery notifications).