</div>
<div class="row">
  <div class="column">
    <div class="thumbnail">
        <img src="CS1.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS2.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS3.jpg" alt="Counter Strike: Global Offensive Screenshot">
      </div>
  </div>
  <div class="column">
      <div class="thumbnail">
        <img src="CS4.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS5.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS6.jpg" alt="Counter Strike: Global Offensive Screenshot">
      </div>
  </div>
  <div class="column">
      <div class="thumbnail">
        <img src="CS7.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS8.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS9.jpg" alt="Counter Strike: Global Offensive Screenshot">
        </div>
  </div>
  <div class="column">
      <div class="thumbnail">
        <img src="CS10.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS11.jpg" alt="Counter Strike: Global Offensive Screenshot">
        <img src="CS12.jpg" alt="Counter Strike: Global Offensive Screenshot">
      </div>
  </div>
</div>

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

.column {
  padding: 0 4px;
}

.column img {
  margin-top: 8px;
  vertical-align: middle;
  display: block;
}