Great question. The accuracy definitely isn't 100%, but it's still pretty good. Gmail now loads images by default, as do most desktop (and mobile) email clients. There are a few exceptions, but overall it's not as bad as it appears.
IIRC, Gmail actually serves cached versions of embedded images using their own domains, which basically means that tracking pixels will get exactly one impression/hit (probably even regardless of the email actually having been opened or not).
