How? You don’t own a (digital) image. You own a token that references an image.
Everyone else can view the image and send it to their friends. But you have some token allegedly signed by the author of the image — who cares?
This is a pretty terrible system, though, as it's completely off-chain and not "Web 3.0" or decentralized in the slightest. The server has to store the "private" content in a private centralized database, so the server owners also can access the content, and any hacker could access the database and publicly dump all of the supposedly token-walled content.
It's just taking advantage of the fact that any computer can determine the Ethereum address that owns a particular token and can check to see if a message was signed by that address's private key.
I could imagine a future standard that could maybe integrate this into Ethereum proper, though I'm not sure if it's technologically possible.
There's no way to prevent someone from just publicly releasing the art, but one could imagine some system like the full resolution piece stored as a public key-encrypted blob in the blockchain which can only be decrypted by a token holder.
Maybe upon token ownership transfers the key could be encrypted with the token buyer's Ethereum address public key, though I'm not sure how or if it could work since I think the original content key would have to stored on the network in some fashion in order to later encrypt the key for token owners. So it might not be possible even in theory, though I don't know nearly enough about Ethereum and cryptography to say with any certainty.
It would instead be like owning a unique piece of paper with a unique number on it, which somehow corresponded to a picture of the autograph. Now the autograph portion is identical between you and anyone else (neither of you has a physical autograph); the only differentiator is your piece of paper. Where is the value?