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HTML 

              
                <!-- .container -->
<div class="container">

  <!-- .gallery-slider -->
  <div class="gallery-slider">

    <!-- __images -->
    <div class="gallery-slider__images">
      <div>
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/700/450" alt="Kitten 1"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/450/700" alt="Kitten 2"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/770/450" alt=""></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/200/210" alt="Kitten 4"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/300/310" alt="Kitten 5"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/770/200" alt="Kitten 6"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/200/770" alt="Kitten 7"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/545/321" alt="Kitten 8"></div>
        </div>
        <!-- /.item -->
      </div>
      <button class="prev-arrow slick-arrow">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1280 1792">
          <path fill="#fff" d="M1171 301L640 832l531 531q19 19 19 45t-19 45l-166 166q-19 19-45 19t-45-19L173 877q-19-19-19-45t19-45L915 45q19-19 45-19t45 19l166 166q19 19 19 45t-19 45z" />
        </svg>
      </button>
      <button class="next-arrow slick-arrow">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1280 1792">
          <path fill="#fff" d="M1107 877l-742 742q-19 19-45 19t-45-19l-166-166q-19-19-19-45t19-45l531-531-531-531q-19-19-19-45t19-45L275 45q19-19 45-19t45 19l742 742q19 19 19 45t-19 45z" />
        </svg>
      </button>
      <span class="caption">&nbsp;</span>
    </div>
    <!-- /__images -->


    <!-- __thumbnails -->
    <div class="gallery-slider__thumbnails">
      <div>
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/700/450" alt="Kitten 1"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/450/700" alt="Kitten 2"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/770/450" alt=""></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/200/210" alt="Kitten 4"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/300/310" alt="Kitten 5"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/770/200" alt="Kitten 6"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/200/770" alt="Kitten 7"></div>
        </div>
        <!-- /.item -->
        <!-- .item -->
        <div class="item">
          <div class="img-fill"><img src="https://placekitten.com/545/321" alt="Kitten 8"></div>
        </div>
        <!-- /.item -->
      </div>

      <button class="prev-arrow slick-arrow">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1280 1792">
          <path fill="#fff" d="M1171 301L640 832l531 531q19 19 19 45t-19 45l-166 166q-19 19-45 19t-45-19L173 877q-19-19-19-45t19-45L915 45q19-19 45-19t45 19l166 166q19 19 19 45t-19 45z" />
        </svg>
      </button>
      <button class="next-arrow slick-arrow">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1280 1792">
          <path fill="#fff" d="M1107 877l-742 742q-19 19-45 19t-45-19l-166-166q-19-19-19-45t19-45l531-531-531-531q-19-19-19-45t19-45L275 45q19-19 45-19t45 19l742 742q19 19 19 45t-19 45z" />
        </svg>
      </button>
    </div>
    <!-- /__thumbnails -->

  </div>
  <!-- /.gallery-slider -->

</div>
<!-- /.container -->
!

CSS 

              
                $light-grey: #e6e6e6;

* {
  box-sizing: border-box;
}
body {
  font-family: Helevetica, Arial, sans-serif;
}

.container {
  max-width: 770px;
}

.gallery-slider {
	position: relative;
	overflow: hidden;
  background-color: $light-grey;

	// __images
	&__images {
		margin: 0;
		position: relative;
		.item {
			padding: 0;
			.img-fill {
				text-align: center;
				padding: 10px;
				height: 200px;
				@media screen and (min-width: 576px) {
					height: 300px;
				}
				@media screen and (min-width: 720px) {
					height: 450px;
				}

				img {
					display: inline-block;
					position: relative;
					max-width: 100%;
					max-height: 100%;
					top: 50%;
					transform: translateY(-50%);
				}
			}
		}
		.next-arrow, .prev-arrow {
			position: absolute;
			top: 50%;
			transform: translateY(-50%);
			width: 47px;
			height: 47px;
			border-radius: 50%;
			background: rgba(0, 0, 0, .70);
			border: 0 none;
			text-align: center;
			color: #FFF;
			z-index: 5;
			opacity: 0;
			transition: all 0.5s;
			outline: 0;
			svg {
				top: 4px;
			}
		}
		.next-arrow {
			right: 25px;
		}
		.prev-arrow {
			left: 25px;
		}
		&:hover .next-arrow, &:hover .prev-arrow {
			opacity: 1;
		}
    .caption {
			width: 100%;
			position: relative;
			text-align: center;
			display: block;
			opacity: 1;
			transition: opacity 0.15s;
			&.hide {
				opacity: 0;
			}
		}
	}

	// __thumbnails
	&__thumbnails {
		position: relative;
		top: auto;
		left: 0px;
		width: 100%;
		z-index: 4;
		transition: all 0.8s;
		margin: 0;
		padding: 13px 0;
		&:before, &:after {
			content: '';
			display: block;
			width: 100px;
			height: 100%;
			position: absolute;
			top: 0;
			z-index: 10;
			pointer-events: none;
		}
		&:before {
			left: 0;
			background: linear-gradient(to right, rgba($light-grey,1) 0%,rgba($light-grey,0) 100%);
		}
		&:after {
			right: 0;
			background: linear-gradient(to right, rgba($light-grey,0) 0%,rgba($light-grey,1) 100%);
		}
		.item {
			.img-fill {
				height: 70px;
				background: white;
				cursor:pointer;
				border: 5px solid white;
				text-align: center;
				img {
					opacity: 0.5;
					transition: all 0.5s;

					display: inline-block;
					position: relative;
					max-width: 100%;
					max-height: 100%;
					top: 50%;
					transform: translateY(-50%);
					
				}
			}
			&:hover .img-fill img {
				opacity: 1;
			}
			&.slick-center {
				img {
					opacity: 1;
				}
			}
		}
		.next-arrow, .prev-arrow {
			position: absolute;
			width: 40px;
			height: 40px;
			top: 50%;
			transform: translateY(-50%);
			background: rgba(0, 0, 0, .60);
			color: #FFF;
			border: 0 none;
			z-index: 15;
			svg {
				top: 2px;
			}
		}
		.next-arrow {
			right: 0px;
		}
		.prev-arrow {
			left: 0px;
		}
	}

	// generic arrows
	.next-arrow, .prev-arrow {
    cursor: pointer;
		svg {
			width: 18px;
			height: 18px;
			position: relative;
		}
	}

	// image fills
	.img-fill {
		width: 100%;
		display: block;
		overflow: hidden;
		position: relative;
		text-align: center
	}

	// slick overrides
	.slick-slider {
		margin: 0 -7.5px;
		padding: 0!important;
	}
	.slick-slide {
		float: left;
		padding: 0 7.5px;
    outline: none;
	}

}
!

JS 

              
                	/*
		variables
	*/

		var $imagesSlider = $(".gallery-slider .gallery-slider__images>div"),
			  $thumbnailsSlider = $(".gallery-slider__thumbnails>div");

	/*
		sliders
	*/

		// images options
		$imagesSlider.slick({
			speed:300,
			slidesToShow:1,
			slidesToScroll:1,
			cssEase:'linear',
			fade:true,
			draggable:false,
			asNavFor:".gallery-slider__thumbnails>div",
			prevArrow:'.gallery-slider__images .prev-arrow',
			nextArrow:'.gallery-slider__images .next-arrow'
		});

		// thumbnails options
		$thumbnailsSlider.slick({
			speed:300,
			slidesToShow:5,
			slidesToScroll:1,
			cssEase:'linear',
			centerMode:true,
			draggable:false,
			focusOnSelect:true,
			asNavFor:".gallery-slider .gallery-slider__images>div",
			prevArrow:'.gallery-slider__thumbnails .prev-arrow',
			nextArrow:'.gallery-slider__thumbnails .next-arrow',
			responsive: [
				{
					breakpoint: 720,
					settings: {
						slidesToShow: 4,
						slidesToScroll: 4
					}
				},
				{
					breakpoint: 576,
					settings: {
						slidesToShow: 3,
						slidesToScroll: 3
					}
				},
				{
					breakpoint: 350,
					settings: {
						slidesToShow: 2,
						slidesToScroll: 2
					}
				}
			]
		});

	/* 
		captions
	*/

		var $caption = $('.gallery-slider .caption');

		// get the initial caption text
		var captionText = $('.gallery-slider__images .slick-current img').attr('alt');
		updateCaption(captionText);

		// hide the caption before the image is changed
		$imagesSlider.on('beforeChange', function(event, slick, currentSlide, nextSlide){
			$caption.addClass('hide');
		});

		// update the caption after the image is changed
		$imagesSlider.on('afterChange', function(event, slick, currentSlide, nextSlide){
			captionText = $('.gallery-slider__images .slick-current img').attr('alt');
			updateCaption(captionText);
		});

		function updateCaption(text) {
			// if empty, add a no breaking space
			if (text === '') {
				text = '&nbsp;';
			}
			$caption.html(text);
			$caption.removeClass('hide');
		}
!
999px

Console