A couple of customers have asked for this feature already. They are not using affiliate CDNs and would like to migrate all their current images to S3 to alleviate load on their main server.

I should add an option to the External Images plugin to allow users to configure Amazon S3 or similar and to use a cron job to migrate the images and update each product/post/custom post type as it progresses.

Similarly, if users/admins then upload images, it should automatically send them to Amazon S3 instead of uploading them to the server.