The way I would describe it is that the primality of a number is almost completely unrelated to these images. The image is chosen beforehand, then a set of inte

The way I would describe it is that the primality of a number is almost completely unrelated to these images. The image is chosen beforehand, then a set of integers is chosen that each looks recognizably close to that image, then that set is searched for one that is prime (which is not unusual). Primality is the chosen attribute simply because it’s widely known and considered to be interesting in popular mathematics. People would be less impressed by images constructed with the binary representation of numbers divisible by 1,000,000, even though that may be a rarer property than primality for the magnitude of numbers considered.