It's actually hard to track down exactly when these images are cached. I don't really trust MailChimp to be truthful about this but I also can't find any specific from Google themselves about whether that caching happens when their SMTP receives the email or when the user actually opens the message.
Since Google's goal with this was clearly to defeat tracking, I would strongly expect the former, but I can't back this up.
http://blog.mailchimp.com/how-gmails-image-caching-affects-o...