They only know when google fetches the image, which can be any time between you receiving it and opening it. I highly doubt it's on the fly right when you open it.
All Gmail does is proxy the request to hide your IP from the server hosting the image file. Gmail does not change the timing of the request, the URL, or the image file.
All Gmail does is proxy the request to hide your IP from the server hosting the image file. Gmail does not change the timing of the request, the URL, or the image file.