if (document.images) {
  var bullet_off = new Image();
  bullet_off.src = "/images/menu_blank.gif";
  var bullet_on = new Image();
  bullet_on.src = "/images/menu.gif";
}

function act(imgName) {
  if (document.images) 
    document[imgName].src = bullet_on.src;
}

function inact(imgName) {
  if (document.images)
    document[imgName].src = bullet_off.src;
}