Why not IPFS for Nostr? When a relay copies a post, they simply need to pin the image, for redundancy. For mobile clients, we can run IPFS-HTTP proxies.
Today I'm officially starting my Nostr journey. I'm not a fan of social media, and already have an abandoned Mastodon profile https://pantherx.social/@franz - but the tech is interesting, and I've got to support what I'm enjoying and working on.