Steganography
“Steganography” originates from the Greek word “steganographia,” a blend of “steganós” (meaning “covered or concealed”) and “- graphia” (meaning “writing”). Cryptography, much like steganography, is a method for hiding information when facing difficult circumstances.   Steganography offers an advantage over just cryptography because the secret message doesn’t draw attention.
Regarding the artwork.  Histiaeus, the ex-ruler of Miletus, was closely monitored by King Darius I in Susa, as documented by Greek historian Herodotus in approximately 499 BC. He intended to dispatch a clandestine communication to his son-in-law, Aristagoras, situated in Miletus. He chose a loyal servant, shaved his head, and marked a (tattoo) message directly onto the skin. Histiaeus concealed the slave until his hair was long enough to obscure the writing. The enslaved person went to Miletus and instructed Aristagoras to shave his head, exposing the message. The hidden tattoo contained a message for Aristagoras, telling him to rebel against Persia.
The Persian Emperor Xerxes I began preparing a colossal army to invade Greece around 482 BC. Exiled Spartan king Demaratus, now residing with the Persians, learned of their invasion plans. He wished to warn Sparta but the heavily guarded roads prevented anyone from leaving the empire undetected. He carved his warning message to the Spartans onto a pair of wooden folding tables.  Afterward, he sealed the tablet and then poured hot, fresh wax onto the wood, coating it. The Persian guards saw the tablet as a harmless wax object, thus the messenger was permitted to enter Greece without issue.