HTML
<main class="cf w-100">
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0006.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0002.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0003.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0004.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0007.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0008.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0009.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0010.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0011.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0012.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0013.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0014.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0015.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0016.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0017.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0018.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0019.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0020.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0021.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
  <div class="fl w-50 w-third-m w-25-ns">
    <div class="aspect-ratio aspect-ratio--7x5">
      <div style="background-image:url(http://mrmrs.github.io/images/0022.jpg);"
        class="bg-center cover aspect-ratio--object"></div>
    </div>
  </div>
</main>
 
css
.aspect-ratio {
    height: 0;
    position: relative;
}
.aspect-ratio--7x5 {
    padding-bottom: 71.42%;
}
.aspect-ratio--object {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 100;
}
.cover {
    background-size: cover !important;
}
.bg-center {
    background-repeat: no-repeat;
    background-position: center center;
}
.cf:before, .cf:after {
    content: " ";
    display: table;
}
.cf:after {
    clear: both;
}
.cf {
    *zoom: 1;
}
.fl {
    float: left;
    _display: inline;
}
.w-50 {
    width: 50%;
}
.w-100 {
    width: 100%;
}
@media screen and (min-width: 30em) {
    .w-25-ns {
        width: 25%;
    }
}
@media screen and (min-width: 30em) and (max-width: 60em) {
    .w-third-m {
        width: calc(100% / 3);
    }
}