<div class="container">
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
  <div class="item">
    <img src="http://www.planwallpaper.com/static/images/kitty-cat.jpg" />
  </div>
</div>

body {
  background-color: #2d2d2d;
}

.container {
  width: 1024px;
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between; 
  background-color: #ffffff;
  padding: 1em;
  margin: 1em auto;
  
  &:after {
    content: "";
    flex: auto;
  }
  
  .item {
    height: 10em;
    width: 25%;
    padding: 0.5em;
    box-sizing: border-box;
    
    img {
      width: 100%;
    }
  }
}