I sized it for a 1x1 key, so I guess it would be an Esc key. Black on yellow or red on yellow are the usual colors. If it was on a 1.25 key, it could be made a bit bigger and more accurately proportioned. Assuming there's sufficient demand for a radiation button...
I think the pirate flag is in higher demand, but it's a more complicated image and I am not an artist. The tape drive is very geometrical, so I just used math.
Hey, anybody want a yellow sign? You'll be just one keystroke away from summoning Cthullu!