I agree that a hash of data != the source data, but these links seem to indicate that it is at least possible, if not already evident, that illegal images and content can be encoded into the bitcoin blockchain (and presumably other blockchains that permit similar writing to the ledger)

JPG of Nelson Mandela encoded into bitcoin blockchain: http://www.righto.com/2014/02/ascii-bernanke-wikileaks-photo...

Illegal porn and classified documents embedded in the blockchain (page 3): https://www.accenture.com/_acnmedia/pdf-33/accenture-editing...

Original research source: https://fc18.ifca.ai/preproceedings/6.pdf