function dailyBackground(node, images) {
    node.style.backgroundImage = "url(\"" + images[Math.floor(Math.random() * images.length)] + "\")";
}
var imgArray = new Array();
imgArray[0] = "/Images/mastheadHome1b_randomised.jpg";
imgArray[1] = "/Images/mastheadHome2b_randomised.jpg";
imgArray[2] = "/Images/mastheadHome3b_randomised.jpg";

dailyBackground(document.body, imgArray);