function preload(i) { 
  var img = new Image(); 
  img.src = i; return img; 
}

preload('/images/menu_bg_a.gif');
preload('/images/menu_right_a.gif');

function rollMenu(name, mode) {
  if ( mode == 'on' ) m = '-a'; else m = '';
  window.document.images[name].src = '/images/menu-' + name + m + '.gif';
}