If someone sells a "NFT" of say, the Monalisa, under what authority is the painting sold digitally? What would stop a random person, who has no connection to ownership of the actual Monalisa from selling this NFT? If there is already a NFT of an image in existence, what would stop someone from changing a small number of pixels of said image, and selling a NFT of said image?

There is also the issue of GANs. Someone could potentially create a GAN whose generator network creates images of art. Once the GAN is created, it would be trivial to create a near unlimited supply of NFTs.

Last summer, millions of Americans were receiving more in unemployment than they were receiving while working and didn't have to pay for expenses related to work such as gas for commuting, dry cleaning, or eating out to lunch. Americans have also received thousands of "stimulus" dollars, while many traditional forms of entertainment are closed. I think the crazy prices of NFTs are largely a result of this excess money given to Americans.