Disclaimer: I wrote this article and made this website.
There was some talk of this issue in the recent fediverse inefficiencies thread. I’m hopeful that in the future we’ll have a decentralized solution for file hosting but for now I deeply believe that users should pay for their own file hosting.
You’re not the first to think about this.
See https://aumetra.xyz/posts/the-fedi-ddos-problem - there an embed server is proposed, to be shared by multiple instances (ideally a great many would use just the one), which can host things like image files and previews.