How to fix image resizing for lightbox?

Question

I’m trying to do that lightbox work on my site, but the images do not resize to their original size, as can be seen below. What can I do about the problem?

function openModal() {
  document.getElementById('myModal').style.display = "block";
}

function closeModal() {
  document.getElementById('myModal').style.display = "none";
}

var slideIndex = 1;
showSlides(slideIndex);

function plusSlides(n) {
  showSlides(slideIndex += n);
}

function currentSlide(n) {
  showSlides(slideIndex = n);
}

function showSlides(n) {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("demo");
  var captionText = document.getElementById("caption");
  if (n > slides.length) {slideIndex = 1}
  if (n < 1) {slideIndex = slides.length}
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";
  }
  for (i = 0; i < dots.length; i++) {
    dots[i].className = dots[i].className.replace(" active", "");
  }
  slides[slideIndex-1].style.display = "block";
  dots[slideIndex-1].className += " active";
  captionText.innerHTML = dots[slideIndex-1].alt;
}

.row > .column {
  padding: 0 8px;
}

.row:after {
  content: "";
  display: table;
  clear: both;
}

.column {
  float: left;
  width: 33.3%;
}

/* The Modal (background) */
.modal {
  display: none;
  position: fixed;
  z-index: 1;
  padding-top: 100px;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: black;
}

/* Modal Content */
.modal-content {
  position: relative;
  background-color: #fefefe;
  margin: auto;
  padding: 0;
  width: 90%;
  max-width: 1200px;
}

/* The Close Button */
.close {
  color: white;
  position: absolute;
  top: 10px;
  right: 25px;
  font-size: 35px;
  font-weight: bold;
}

.close:hover,
.close:focus {
  color: #999;
  text-decoration: none;
  cursor: pointer;
}

.mySlides {
  display: none;
}

/* Next & previous buttons */
.prev,
.next {
  cursor: pointer;
  position: absolute;
  top: 50%;
  width: auto;
  padding: 16px;
  margin-top: -50px;
  color: white;
  font-weight: bold;
  font-size: 20px;
  transition: 0.6s ease;
  border-radius: 0 3px 3px 0;
  user-select: none;
  -webkit-user-select: none;
}

/* Position the "next button" to the right */
.next {
  right: 0;
  border-radius: 3px 0 0 3px;
}

/* On hover, add a black background color with a little bit see-through */
.prev:hover,
.next:hover {
  background-color: rgba(0, 0, 0, 0.8);
}

/* Number text (1/3 etc) */
.numbertext {
  color: #f2f2f2;
  font-size: 12px;
  padding: 8px 12px;
  position: absolute;
  top: 0;
}

.caption-container {
  text-align: center;
  background-color: black;
  padding: 2px 16px;
  color: white;
}

img.demo {
  opacity: 0.6;
}

.active,
.demo:hover {
  opacity: 1;
}

img.hover-shadow {
  transition: 0.3s
}

.hover-shadow:hover {
  box-shadow: 0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19)
}

<div role="tabpanel" class="tab-pane" id="messages">
<div class="trip-tab-gallery">
<ul>
  <div class="column">
    <li><img src="images/tab-gallery-img-1.jpg" onclick="openModal();currentSlide(1)" class="hover-shadow">
  </li>
  </div>
  <div class="column">
    <li><img src="images/tab-gallery-img-2.jpg" onclick="openModal();currentSlide(2)" class="hover-shadow">
  </li>
  </div>
  <div class="column">
    <li><img src="images/tab-gallery-img-3.jpg" onclick="openModal();currentSlide(3)" class="hover-shadow">
  </li>
  </div>
  <div class="column">
    <li><img src="images/tab-gallery-img-4.jpg" onclick="openModal();currentSlide(4)" class="hover-shadow">
  </li>
  </div>
  </ul>

<div id="myModal" class="modal">
  <span class="close cursor" onclick="closeModal()">&times;</span>
  <div class="modal-content">

    <div class="mySlides">
      <div class="numbertext">1 / 4</div>
        <img src="images/tab-gallery-img-1.jpg" style="width:100%">
    </div>

    <div class="mySlides">
      <div class="numbertext">2 / 4</div>
        <img src="images/tab-gallery-img-2.jpg" style="width:100%">
    </div>

    <div class="mySlides">
      <div class="numbertext">3 / 4</div>
        <img src="images/tab-gallery-img-3.jpg" style="width:100%">
    </div>

    <div class="mySlides">
      <div class="numbertext">4 / 4</div>
        <img src="images/tab-gallery-img-4.jpg" style="width:100%">
    </div>

    <a class="prev" onclick="plusSlides(-1)">&#10094;</a>
    <a class="next" onclick="plusSlides(1)">&#10095;</a>

    <div class="caption-container">
      <p id="caption"></p>
    </div>

    <div class="column">
      <img class="demo" src="images/tab-gallery-img-1.jpg" onclick="currentSlide(1)" alt="Nature">
    </div>

    <div class="column">
      <img class="demo" src="images/tab-gallery-img-2.jpg" onclick="currentSlide(2)" alt="Trolltunga">
    </div>

    <div class="column">
      <img class="demo" src="images/tab-gallery-img-3.jpg" onclick="currentSlide(3)" alt="Mountains">
    </div>

    <div class="column">
      <img class="demo" src="images/tab-gallery-img-4.jpg" onclick="currentSlide(4)" alt="Lights">
    </div>
  </div>
</div>
</div>
</div>

full page here