There are no blockchains that store pictures.
This is one of the original old-school arguments politicians use for new technologies that most ppl don't understand.

1. Terrorists
2. Drug Dealers
3. Pedophiles

The only reason to post instructions on a blockchain about how to access child porn is to try and discredit the blockchain in a psyop. There's no practical reason to use a public platform to advertise something that illegal (other than fabricating propaganda by capitalizing on ignorance and irrationality). If someone wants to use blockchain to advertise publicly a very illegal thing they are doing... doesn't that make them way easier to catch? So we actually want that to be happening, right? Any tool can be used to commit a crime. Using that as an excuse to say "this is why we shouldn't have this tool" or "this is why I should be in charge of regulating that tool" is transparently invalid.