I came to say that images.com is also blocked in Iran.

Until the next fact

Doesn't this mean that Alice could:

A: ddos Bob's media source and harvest the ip addresses etc of Bob's followers.

B: ddos Bob's media source and spread false information by high jacking Bob's media.

What could be the main use case of this backup?

You got to hash the image so no one can alter the original intent or add exploits to the image. The hash can be looked up on cacheable media, e.g. IPFS

If URIs were wrapped as a magnet 🧲 then additional mirrors could simply add to the URI array.

This would ensure copies share the same hash,

The bonus is clients could also act as mirrors in a swarming scenario with WebRTC