Javascript兑现图片随机漂浮

<html>
<head>
</head>
<body>
	<div id="img" style="position:absolute;width:200; height:151"><img src="pic.jpg" border="0"></div>

<script LANGUAGE="JavaScript">
var xPos = 20;
var yPos = 10;
img.style.left= xPos;
img.style.top = yPos;
var step = 1;
var delay = 30; 
var width,height,Hoffset,Woffset;
var y = 1;
var x = 1;
var interval;
img.visibility = "visible";
function changePos() 
{
  width = document.body.clientWidth;
  height = document.body.clientHeight;
  Hoffset = img.offsetHeight;
  Woffset = img.offsetWidth;
  if (y) 
  {
    yPos = yPos + step;
  }
  else 
  {
    yPos = yPos - step;
  }
  if (yPos < 0) 
  {
    y = 1;
    yPos = 0;
  }
  if (yPos >= (height - Hoffset)) 
  {
    y = 0;
    yPos = (height - Hoffset);
  }
  if (x) 
  {
    xPos = xPos + step;
  }
  else 
  {
    xPos = xPos - step;
  }
  if (xPos < 0) 
  {
    x = 1;
    xPos = 0;
  }
  if (xPos >= (width - Woffset)) 
  {
    x = 0;
    xPos = (width - Woffset);
  }
  img.style.left = xPos + document.body.scrollLeft;
  img.style.top = yPos + document.body.scrollTop;
}
function start() 
{
  interval = setInterval('changePos()', delay);
}
function pause_resume() 
{
    clearInterval(interval);
}
start();
</script>
</body>
</html>