function swapImage(id, zustand) {
  var image = document.getElementById(id);
  switch(zustand) {
    case 'on':
      image.src = 'fileadmin/asw/templates/pics/nav/' + id + '-2.jpg';
      break;
    case 'off':
      image.src = 'fileadmin/asw/templates/pics/nav/' + id + '-1.jpg';
      break;
    default:
      image.src = 'fileadmin/asw/templates/pics/nav/' + zustand;
  }
}