document.querySelector('img.preview').addeventListener('function(e) {
  var lowRes = e.target.src;
  var myOverlay = document.querySelector('.overlay');
  var HighRes = document.createElement('img');
  var mySpinner = document.createElement('img');
  
  myOverlay.style.display = 'block';
  highRes.className = 'bgImg';
  highRes.src = lowRes.substr(0,lowres.length-7) + '.jpg';
  myOverlay.appendChild(highRes);
  
  mySpinner.className = 'spinner';
  mySpinner.src = "images/spinner.gif";
  myOverlay.appendChild(mySpinner);
  
  highRes.addEventListener('load', function() {
    mySpinner.parentNode.removeChild(mySpinner);
  });
}, false);