var preloaded = new Array();
function preload_images() {
  for (var i = 0; i < arguments.length; i++) {
    preloaded[i] = document.createElement('img');
    preloaded[i].setAttribute('src', arguments[i]);
  };
};

preload_images(
  '/App_Themes/PortalSF/Images/MenuTop/menuItem1.jpg',
  '/App_Themes/PortalSF/Images/MenuTop/menuItem2.jpg',
  '/App_Themes/PortalSF/Images/MenuTop/menuItem3.jpg'
);