Funny you should ask. I just saw this today with Gossip client. It wouldn’t render an image because of hash mismatch. Amethyst also does something similar, showing a red ! icon if the image has been altered post-upload.
Discussion
Nice! That’s exactly what I hoped it would do! That’s truly impressive.
Can you screenshot of an example when you stumble upon one of those? Very curious to see how it looks.
Here’s how it looks on Amethyst. On Gossip it’s just a highlighted text in brackets that says it won’t load image due to hash mismatch. 
Wow! Impressive.
So this is taking the note content where it already has the hash of the blossom file, comparing it with the hash of the media delivered, and detecting if they match right?
I ask because I would like to be able to explain it to others.
It wouldn’t prove ownership per se on a broader Nostr perspective, but the hash would be tied to the pubkey that uploaded it to the blossom server. So, only that blossom server or any mirrors could tie it to the pubkey. Other than Amethyst’s warning, there wouldn’t be a way to tell if any media was doctored. But, Amethyst’s feature helps in the event someone on the hosting side changed the image after it was added to the note.