I agree they could do it, but people upthread are suggesting that Google doesn't do that, and only loads the image when you open the message.
they could track if an email was opened twice
first one = Google
second one = user
I would imagine that google caches the images so there would be one request instead of two.
That would essentially render open statistics meaningless and would let Google cripple another industry after the promotions tab and 'not provided.'
I really hope they don't because it's such valuable information when creating email copy...