NFTs are usually referred to as picture on-chain, but NFT is more than just on-chain JPEG. NFTs on Ethereum are built on smart contracts which are standardized in most cases as ERC-721, sometimes referred to as ERC-1155. If you do lack understanding of what ERC means, you can read this article in order to understand ERCs and it's fundamentals. This article is deep dive and overview of Metadata and issues the NFT ecosystem is facing with metadata.