There are apparently two methods:
- in the script, e.g. as in the Genesis block message
- in output addresses
- http://eternitywall.it/ Eternity WallLaunched 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".