function switchImg(oldElem, newElem) {
  document.getElementById(oldElem).style.visibility = 'hidden';
  document.getElementById(newElem).style.visibility = 'visible';
}

function switchAll(newElem) {
  document.getElementById('bert1').style.visibility = 'hidden';
  document.getElementById('bert2').style.visibility = 'hidden';
  document.getElementById('bert3').style.visibility = 'hidden';
  document.getElementById('bert4').style.visibility = 'hidden';
  document.getElementById(newElem).style.visibility = 'visible';
}