buttonImg = new Array();

  for (i=0; i<=4; i++) {
    buttonImg[i] = new Image();
    buttonImg[i].src = "image/button_" + i + "_on.gif";
  }

  function imageOn(n){
    document.images['menu'+n].src = buttonImg[n].src
  }

  function imageOff(n){
    document.images['menu'+n].src = "image/button_" + n + "_off.gif";
  }