function returnObjById(id) {
  if (document.getElementById)
    var returnVar = document.getElementById(id);
    else if (document.all)
      var returnVar = document.all[id];
    else if (document.layers)
      var returnVar = document.layers[id];
    return returnVar;
}

function hideImage (id) {
 returnObjById(id).style.visibility = 'hidden';
}

function unhideImage (id) {
 returnObjById(id).style.visibility = 'visible';
}