Attendee Question: Does the picture example really work? What prevents anyone from creating a copy of the picture outside of the blockchain? If the picture has any value outside the blockchain, it will retain this value whether or not it's part of the blockchain.
Answer (Toufi Saliba): Well, if you take the cryptographic hash of the picture and insert it into the current block, it becomes as immutable as the entire system. Historic events, i.e., smart contracts (like transactions) are hashed and concat-hash through temporal coupling in an algorithm that builds up the Merkle Tree, then its daisy chained through future merkle trees, i.e. hash of pervious block or merklroot is added to existing and then together into the future block, etc. If you change a single value in the past, the entire present values will change, let alone that the computation needed to PoW or whatever consensus mechanism you use is very hard to replicate. However Pictures, even if you simply modify the color in [a way] unnoticeable to the human eye or even if you put one extra pixel, the entire hash will change. So not only the hash is very hard to create a collision (almost impossible), [but also] inserting it in the history of the blockchain becomes more challenging.