We've got some great ideas, here.
I'd start with "Imagine a World Without Guns" at the top of a picture like this:
"A large hand around a small throat." Or even better, a smallish woman, her face a little bruised, cowering before a large, well-built man, both unarmed. Preferably, they would be in some sort of period garb, to remind the viewer that we've already tried a world without guns.
And at the bottom, "The gun IS EQUALITY. The gun IS CIVILIZATION."