My suggestion would be a CDN with maybe Azure. If you're the owner of all the sites, then it's not a big deal. If you are not the owner of all the sites, then this becomes a problem, no matter what vehicle you use as your media sharing tool because you won't be able to set permissions to restrict one website owner from accessing another owners information.
If using an Asure CDN, all sites or instances would essentially connect to the same CDN and store their files there. I've not tested this out but hypothetically this should work.