The difference is that video games have a way to enforce ownership. CS:GO skins are valuable because Valve doesn’t let you just upload your own skins.
I can download any NFT and use it as my own profile picture.
I can download any NFT and use it as my own profile picture.
Sure, or if they check the images you upload (YouTube does this on a massive scale). Or if they show a checkmark on verified images. Or if they prevent uploading unverified profile images at all (presumably this is how Valve games work, so that everyone can't just download a copy of the expensive item and use it online).
> But the latter still only works if the general population of Twitter thinks it’s cool.
Yes, of course, but as you explain, that's no different than with video games, especially if the items are purely cosmetic.
If an app wanted to do this in a decentralized way then they'd likely have Kleros curate a list of known NFT sets or use a standard like TokenLists where a user can subscribe to whatever curated list of known NFTs they wanted.
You cannot, unless you have a vastly different understanding of what an NFT is than what it actually is. A NFT is not a picture, it's a cryptocurrency that only has one item. You could download the data representing a NFT (it's just ascii data), but not sure what you'd get out of that.