window.onload = function() { document.body.style.backgroundImage = "url(" +  RandomImage() + ")"; }

function RandomImage() {
    var myImages = new Array();
    myImages[0] = "/media/images/backgrounds/bg.jpg";
    myImages[1] = "/media/images/backgrounds/bg1.jpg";
    myImages[2] = "/media/images/backgrounds/bg2.jpg";
    myImages[3] = "/media/images/backgrounds/bg3.jpg";
    return myImages[Math.floor(Math.random() * myImages.length)]; 
    }