var speed = 50;
var stopScript = false;
var opacity = 40;

function moveback(movert,movedn,hPos,vPos) {
  if(stopScript)return;
  vPos -= movert;
  if (vPos < -25) speed += 5;
  document.getElementById('background_image').style.backgroundPosition= hPos + "px " + vPos + "px "
  if(document.all) {
		document.getElementById('background_image').style.filter = "alpha(opacity=" + opacity + ")";
	} else {
		document.getElementById('background_image').style.MozOpacity = opacity/100;
	}
  if (vPos == -40) stopScript = true;
  opacity+=2;
  setTimeout("moveback("+movert+","+movedn+","+hPos+","+vPos+")",speed)
}