If images are not loading because Amazon Cloudfront was blocked in the email at that time, there are most uploaded images will have the following URL:
https://d15k2d11r6t6rl.cloudfront.net/*
So when you whitelist this domain it will prevent the blocking of image links.
Note: We cannot promise that things won't change in the future. Should it occur, you'll observe a change in the URLs and will have to modify your rules accordingly.