Sys.Application.add_load(function() {
    var images = new Array();
    images[0] = "masthead1.jpg";
    images[1] = "masthead2.jpg";
    images[2] = "masthead3.jpg";
    images[3] = "masthead4.jpg";
    images[4] = "masthead5.jpg";
    var index = Math.floor(Math.random() * images.length);
    var masthead = $get("masthead");
    masthead.style.backgroundImage = "url(App_Themes/Theme1/Images/" + images[index] + ")";
});