<section class="gallery">
  <div class="gallery__item">
    <input type="radio" id="img-1" checked name="gallery" class="gallery__selector"/>
    <img class="gallery__img" src="https://i.picsum.photos/id/1015/600/400.jpg" alt=""/>
    <label for="img-1" class="gallery__thumb"><img src="https://i.picsum.photos/id/1015/150/100.jpg" alt=""/></label>
  </div>
  <div class="gallery__item">
    <input type="radio" id="img-2" name="gallery" class="gallery__selector"/>
    <img class="gallery__img" src="https://i.picsum.photos/id/1039/600/400.jpg" alt=""/>
    <label for="img-2" class="gallery__thumb"><img src="https://i.picsum.photos/id/1039/150/100.jpg" alt=""/></label>
  </div>
  <div class="gallery__item">
    <input type="radio" id="img-3" name="gallery" class="gallery__selector"/>
    <img class="gallery__img" src="https://i.picsum.photos/id/1057/600/400.jpg" alt=""/>
    <label for="img-3" class="gallery__thumb"><img src="https://i.picsum.photos/id/1057/150/100.jpg" alt=""/></label>
  </div>
  <div class="gallery__item">
    <input type="radio" id="img-4" name="gallery" class="gallery__selector"/>
    <img class="gallery__img" src="https://i.picsum.photos/id/106/600/400.jpg" alt=""/>
    <label for="img-4" class="gallery__thumb"><img src="https://i.picsum.photos/id/106/150/100.jpg" alt=""/></label>
  </div>
</section>

* {
  box-sizing: border-box;
}

img {
  max-width: 100%;
  vertical-align: top;
}

.gallery {
  display: flex;
  margin: 10px auto;
  max-width: 600px;
  position: relative;
  padding-top: 66.6666666667%;
}
@media screen and (min-width: 600px) {
  .gallery {
    padding-top: 400px;
  }
}
.gallery__img {
  position: absolute;
  top: 0;
  left: 0;
  opacity: 0;
  transition: opacity 0.3s ease-in-out;
}
.gallery__thumb {
  padding-top: 6px;
  margin: 6px;
  display: block;
}
.gallery__selector {
  position: absolute;
  opacity: 0;
  visibility: hidden;
}
.gallery__selector:checked + .gallery__img {
  opacity: 1;
}
.gallery__selector:checked ~ .gallery__thumb > img {
  box-shadow: 0 0 0 3px #0be2f6;
}