var xPos = 20;
var yPos = 400
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();