var xPos = 300;
var yPos = 200;
var step = 1;
var delay = 10;
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = true;
var xon = 0;
var pause = true;
var interval;
var diffY = 0;
var diffX = 0;
img1.style.top = yPos;
function changePos() {
    height = document.documentElement.clientHeight;
    width = document.documentElement.clientWidth;
    if (document.documentElement && document.documentElement.scrollTop) {
        diffX = document.documentElement.scrollLeft;
        diffY = document.documentElement.scrollTop;
    }
    else if (document.body) {
        diffX = document.body.scrollLeft;
        diffY = document.body.scrollTop;
    }
    Hoffset = img1.offsetHeight;
    Woffset = img1.offsetWidth;
    img1.style.left = xPos + diffX + "px";
    img1.style.top = yPos + diffY + "px";
    if (yon)
    { yPos = yPos + step; }
    else
    { yPos = yPos - step; }
    if (yPos < 0)
    { yon = true; yPos = 0; }
    if (yPos >= (height - Hoffset))
    { yon = false; 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() {
    img1.visibility = "visible";
    interval = setInterval('changePos()', delay);
}
function pause_resume() {
    if (pause) {
        clearInterval(interval);
        pause = false;
    }
    else {
        interval = setInterval('changePos()', delay);
        pause = true;
    }
}
start();