var xPos = 20;
    var yPos = document.body.clientHeight;
    var step = 1;
    var delay = 30; 
    var height = 0;
    var Hoffset = 0;
    var Woffset = 0;
    var yon = 0;
    var xon = 0;
    var pause = true;
    var interval;
    img.style.top = yPos;
    function changePos() {
    width = document.body.clientWidth;
    height = document.body.clientHeight;
    Hoffset = img.offsetHeight;
    Woffset = img.offsetWidth;
    img.style.left = xPos + document.body.scrollLeft;
    img.style.top = yPos + document.body.scrollTop;
    if (yon) {
    yPos = yPos + step;
    }
    else {
    yPos = yPos - step;
    }
    if (yPos < 0) {
    yon = 1;
    yPos = 0;
    }
    if (yPos >= (height - Hoffset)) {
    yon = 0;
    yPos = (height - Hoffset);
    }
    if (xon) {
    xPos = xPos + step;
    }
    else {
    xPos = xPos - step;
    }
    if (xPos < 0) {
    xon = 1;
    xPos = 0;
    }
    if (xPos >= (width - Woffset)) {
    xon = 0;
    xPos = (width - Woffset);
      }
    }
    function start() {
    img.visibility = "visible";
    interval = setInterval('changePos()', delay);
    }
    start();