main.js

var thumbs = document.querySelectorAll('.thumb');
var mainImg = document.querySelector('.main');

for(i = 1; i <= thumbs.length ; i++) {
  var requestObj = 'images/pic' + i + '.jpg';
  retrieveImage(requestObj,i-1);
}

function retrieveImage(requestObj,imageNo) {
  var request = new XMLHttpRequest();
  request.open('GET', requestObj, true);
  request.responseType = 'blob';
  request.send();

  request.onload = function() {
    var objectURL = URL.createObjectURL(request.response);
    thumbs[imageNo].setAttribute('src',objectURL);
    thumbs[imageNo].onclick = function() {
      mainImg.setAttribute('src',objectURL);
      mainImg.className = 'blowup';
        for(i = 0; i < thumbs.length; i++) {
          thumbs[i].className = 'thumb darken';
        }
    }
  }
}

mainImg.onclick = function() {
  mainImg.className = 'main';
  for(i = 0; i < thumbs.length; i++) {
    thumbs[i].className = 'thumb';
  }
}