Event.onDOMReady(function() {

// choose random number in range
var randomNum = function(low, high) { return Math.floor(Math.random() * (1 + high - low) + low); }

// choose random array value
var getRandom = function(arr) { return arr[randomNum(0, arr.length - 1)]; }

var dir = "site/images/headers/";
var arr = ["1.jpg", "2.jpg", "3.jpg", "4.jpg"];

$("header").down("h1").style.backgroundImage = "url(" + dir + getRandom(arr) + ")";

});