var xPos = 20;
var yPos = document.body.clientHeight;
var step = 2;
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);
         }
     interval = setTimeout('changePos()',delay);
	 }

function start() {
img.visibility = "visible";
interval = setInterval('changePos()', delay);
}

function stop()
{
 
clearTimeout(interval);
}

changePos();

//start();