Why you can't just screenshot an NFT

The problem with NFTs... well, one of the problems, is that they might be a certificate but they're a certificate signed by <unknown> about <subject to change>. There is no cryptographic proof that the issuer of the NFT is actually the author of the artwork and there is usually no checksum over the artwork itself, let alone having the artwork on the blockchain. So what's the point? If anyone can issue an NFT on anyone's art and even change the art it points to afterwards, it's really a botched attempt at creating this kind of universal art ledger.