I was going to suggest this as well, which is why I searched and found this topic.
An NFT tag is worth considering not only because it would indicate the availability of a picture as an NFT -personally I'm really not into them but others here might be, and it'll be useful to them in the same way reward_available is to others- but more importantly because it would indicate that a specific version of an image is most probably the best version there is, i.e. the largest in resolution and quality that will be made available by the artist.
This might even encourage people to try and find more NFT-type sources for their uploads rather than the usual ones like pixiv, giving us the best versions of pictures in the process.
It's tricky topic to tag because while there are artists who do release NFTs through opensea or foundation, there are a lot of artists we have posts of who are actively anti-NFT and had their work minted by a third party. We'd have to make sure it was by the artist themself and not a third party if we were to start adding those tags.
That's essentially just art theft, and we're already supposed to be wary of that and try our best to properly tag and source the original artist. If anything, I'd argue the best chance a stolen piece of artwork like that has of being identified and properly attributed to its original creator, is by getting it uploaded and tagged here.