https://wordpress.org/plugins/amazon-s3-and-cloudfront/ (most popular and easiest plugin for S3 Image storage)
With upfront + smush + hummingbird not working in multi-site network.
Upfront themes generate images that are not "smushed/compressed" properly giving negative scores on the google page speed test (which effects page rank)
Smush with + S3 plugin works on other themes
Upfront + S3 works
But having upfront + s3 + smush is not working.
Who ever coded upfront generates image on the front end, but does not properly leave the images in the image library. I'm gonna say it...this BAD code does two things:
1. If you delete an image from the upfront front end...that image stays on the server with no front end way of being deleted. Its not gonna be in the image library, its just gonna stay on the server forever taking up space.
2. Because the upfront images are not stored in the library, I'm guessing that throws off smush with S3 because they both rely on being used with proper wordpress functions.
Please test....very popular and important plugin for creating a scalable system.