At this moment in time, there are no laws or norms prohibiting this at the moment. The regulations are determined by the marketplace you use to generate and sell NFT pictures. Some websites have rules, while others do not. Remember that behind NFTs value is tied to the work's uniqueness and scarcity. It's important to remember that NFT buyers are, in theory, collectors. They are the ones who determine the value of a piece by deciding to buy it. For this reason, I personally think that there won't be such a law anytime soon and, therefore, you are free to sell your work both as stock photography and as an NFT photo.