function ChangeBgImg(imgs) {
    bdy = document.getElementsByTagName("body");
    bdy[0].style.backgroundImage = 'url('+imgs+')';
    bdy[0].style.backgroundPosition = "50% top";
    bdy[0].style.backgroundRepeat = "no-repeat";
}

function ChangeBgImgInit() {
    bdy = document.getElementsByTagName("body");
    bdy[0].style.width = "100%";
    bdy[0].style.height = "100%";
    bdy[0].style.margin = "0px";
    bdy[0].style.backgroundImage = 'url(bg)';
}

onload=ChangeBgImgInit;