var preload = new Array(
  'media/menu-book-over.jpg',
  'media/menu-author-over.jpg',
  'media/menu-guide-over.jpg',
  'media/menu-media-over.jpg',
  'media/menu-buzz-over.jpg',
  'media/menu-library-over.jpg',
  'media/menu-lobby-over.jpg'
  );

var loader = new Array();
for(var i = 0; i < preload.length; i++){
   loader[i] = new Image();
   loader[i].src = preload[i];
}

function MOver(imageid) {
  image = document.getElementById(imageid);
  image.src = "media/menu-" + imageid + "-over.jpg";
}

function MOut(imageid) {
  image = document.getElementById(imageid);
  image.src = "media/menu-" + imageid + ".jpg";
}