It would be great to set some browser caching headers on those images, currently those seem to refetched every time - at least from what I can tell. Does S3 have a setting for that?

Does the image URL change when user uploads a new version? If yes, then you could essentially cache those forever:

cache-control: public, max-age=31536000

It would also probably be good idea to gzip (or ideally brotli) those images on response ;)