There are apparently two methods:
* in the script, e.g. as in the <Genesis block message>
* in output addresses

Specific implementations:
* http://eternitywall.it/ Eternity Wall

  Launched 2015 https://www.newsbtc.com/news/bitcoin/eternity-wall-records-1150-documents-blockchain-first-year/

  TODO find sample transactions. Did it support images?

  Shutdown sometime after 2019, working archive: https://web.archive.org/web/20190417074034/https://eternitywall.it/ says "Sorry, the service is not properly working at the moment..." and last working message timestamped "April 16, 2019 8:02 PM GMT".

Bibliography:
* https://bitcoin.stackexchange.com/questions/32575/what-methods-are-currently-used-to-embed-additional-data-into-the-bitcoin-blockc
* https://bitcoin.stackexchange.com/questions/39347/how-to-store-data-on-the-blockchain


How to store data in the Bitcoin blockchain