Earlier quoted context omitted.
You can't really put content on the blockchain, that costs a surprising amount because of all the proof-of-waste. What happens is people put the content on a host and the URL on the blockchain. I suppose in theory it could be kept in IPFS, but few NFTs seem to bother with that.
Read the linked parent forum post. They are talking exactly about putting the actual images on the blockchain.
Browse through https://opensea.io/, the most prominent NFT marketplace at the moment. Digging through the DOM of their website will reveal a lot of links to quite standard content storage solutions. They usually go through a bit of trouble to prevent people from simply clicking "view image in new tab" to reveal the actual location, but it's not difficult to get around.