Technically yes you could use base64 as your url in the image field, however, I wouldn't recommend it unless it's an SVG or something small (a few KBs) since th

Technically yes you could use base64 as your url in the image field, however, I wouldn't recommend it unless it's an SVG or something small (a few KBs) since the metadata cannot be cached or asynchronously fetched by browsers on page load. So a massive metadata file could cause issues viewing your NFT in most marketplaces or galleries. There are other fields you can add to meet marketplace standards, such as OpenSea's: https://docs.opensea.io/docs/2-adding-metadata https://docs.opensea.io/docs/2-adding-metadata To learn more about ERC721 highly recommend the auditing firm OpenZeppelin's implementation. It's one of the most used and battle tested: https://docs.openzeppelin.com/contracts/3.x/erc721 https://docs.openzeppelin.com/contracts/3.x/erc721