Галерея с миниатюрами изображений на js

var slideIndex = 1;
showSlides(slideIndex);


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

function showSlides(n) {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("demo");
  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";
}

* {
  box-sizing: border-box;
}
.container {
  position: relative;
}
.mySlides {
  display: none;
}
.cursor {
  cursor: pointer;
}
.row:after {
  content: "";
  display: table;
  clear: both;
}
.column {
  float: left;
  width: 16.66%;
}
.demo {
  opacity: 0.6;
}
.active,
.demo:hover {
  opacity: 1;
}

<div class="container">
  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_lights_wide.jpg" style="width:100%">
  </div>

  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_5terre_wide.jpg" style="width:100%">
  </div>

  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%">
  </div>

  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%">
  </div>

  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_5terre_wide.jpg" style="width:100%">
  </div>

  <div class="mySlides">
      <img src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%">
  </div>

  <div class="row">
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_lights_wide.jpg" style="width:100%" onclick="currentSlide(1)" alt="The Woods">
    </div>
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_5terre_wide.jpg" style="width:100%" onclick="currentSlide(2)" alt="Cinque Terre">
    </div>
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%" onclick="currentSlide(3)" alt="Mountains and fjords">
    </div>
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%" onclick="currentSlide(4)" alt="Northern Lights">
    </div>
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_5terre_wide.jpg" style="width:100%" onclick="currentSlide(5)" alt="Nature and sunrise">
    </div>
    <div class="column">
      <img class="demo cursor" src="https://www.w3schools.com/howto/img_mountains_wide.jpg" style="width:100%" onclick="currentSlide(6)" alt="Snowy Mountains">
    </div>
  </div>
</div>