In a Discord discussion I discovered that the images of Cardano NFT are not on the Cardano network but on IPFS and that we need to pin them or else they might disappear.
How can I do that?
It is true most of the NFTs on Cardano are not storing their media directly on the Cardano blockchain, mostly due to the cost, as storing something on a blockchain (that is being replicated and validate by thousands participants) is expensive.
Instead, they store their metadata on Cardano blockchain and one part of this metedata is a pointer to a storage location of the media. The most popular storage for these media is IPFS (InterPlanetary File System) which is a peer-to-peer network for storing and sharing data on a distributed file system.
However, for the media to be present on IPFS at least one participant (node) on the network must hold the file on their hard drive. This is what we're calling pinning. You opt to store the file in question on your hard drive and make it accessible to others. Someone has to do it, else the file disappears forever from the IPFS network.
So who is responsible to store it? The creator of your NFT project should do it and it would be prudent to investigate if your NFT project is doing that.
That being said, at Blockfrost we're at the moment pinning all Cardano NFTs that are on-chain at the moment. A service to the community we sadly might not be able to continue for long due to raising costs.
Also, if your project used a service like NMKR Studio, they are pinning the media minted using their service.