function setRandomBackground()
{
    var bgImg = new Image();
    var rndNum=Math.floor(Math.random()*44);
    bgImg.onLoad = bgLoaded(rndNum);
    bgImg.src = "images/backgrounds/" + rndNum + ".jpg";
    bgImg.width = 800;
    bgImg.height = 600;
}

function bgLoaded(rndNum) {
    var eBody;
    var elems = document.getElementsByTagName("body");
    eBody = elems[0];
    eBody.style.backgroundImage = "url('images/backgrounds/" + rndNum + ".jpg')";
}