<div class="row">
  <div class="column">
    <img src="https://images.pexels.com/photos/1108099/pexels-photo-1108099.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/38867/pexels-photo-38867.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/406014/pexels-photo-406014.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/1378849/pexels-photo-1378849.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />
  </div>
  <div class="column">
    <img src="https://images.pexels.com/photos/1389994/pexels-photo-1389994.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/1254140/pexels-photo-1254140.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/320014/pexels-photo-320014.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />
  </div>
  <div class="column">
    <img src="https://images.pexels.com/photos/39317/chihuahua-dog-puppy-cute-39317.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/1805164/pexels-photo-1805164.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/1452717/pexels-photo-1452717.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />

    <img src="https://images.pexels.com/photos/129634/pexels-photo-129634.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" alt="" />
  </div>
</div>

.row {
  display: flex;
  flex-wrap: wrap;
  padding: 0 4px;
}

.column {
  flex: 25%;
  max-width: 33.3%;
  padding: 0 4px;
}

.column img {
  margin-top: 8px;
  vertical-align: middle;
  width: 100%;
  filter: grayscale(1) brightness(0.5);
  border-radius: 5px;
  cursor: pointer;
  transition: 0.3s linear;
}

.column img:hover {
  filter: grayscale(0);
}

@media screen and (max-width: 700px) {
  .column {
    flex: 100%;
    max-width: 100%;
  }
  .column img {
    filter: grayscale(0) brightness(1);
  }
}