In many cases you don't even need to modify it. Ownership of an NFT often does not provide copyright on the original image (https://www.wipo.int/wipo_magazine/en/2021/04/article_0007.h... has some interesting details on what you do and don't get with an NFT). So if you made the original image, you can just mint a new NFT of that image and sell it.

Heck you can mint as many NFTs as you like of any image you own the copyright of :)