function preLoadImg (iURL)
{
  img = new Image();
  img.src = iURL;
  return img;
}

function swapImg ( bName, sourceImg) {
  document[bName].src = sourceImg.src;
  return true;
}

function dropMenu(naviID) {
  var ref = document.getElementById(naviID);
  
  ref.style.visibility = 'visible';
}

function hideMenu(naviID) {
  var ref = document.getElementById(naviID);
  
  ref.style.visibility = 'hidden';
}