This is a fairly simple response changes the background of the site with a list of items
function randomToN(maxVal) {
var randVal = Math.random() * maxVal;
return typeof 0 == 'undefined' ? Math.round(randVal) : randVal.toFixed(0);
};
var list = [ "IMG0.EXT", "IMG2.EXT","IMG3.EXT" ], // Images
ram = list[parseFloat(randomToN(list.length))], // Random 1 to n
img = ram == undefined || ram == null ? list[0] : ram; // Detect null
$("div#ID").css("backgroundImage", "url(" + img + ")"); // push de background