As far as I remember, Gmail used to prefetch images to prevent senders learning if and when recepient opens an email, but if this behaviour changed, I didn't know that.
But the image request still happens at the time you open the email. Google does not prefetch the images in unopened emails.
And if the image URL is personalized, it can still be correlated with your email address by the sender to record an open. Google does not try to guess which part of the URL they can dump without breaking the image.