function getPosition(obj) {
  var leftValue = 0;
  while(obj) {
	leftValue += obj.offsetLeft;
	obj = obj.offsetParent;
  }
  return leftValue;
}

function storeImg() {
  var pos = getPosition(document.getElementById('nav7'));
  pos = pos - 40;
  document.getElementById('storepointer').style.left = pos + "px";
  document.getElementById('storepointer').style.display = 'block';
}