.box__in {    

  background-image: url("http://satyr.io/320x320");  

  background-size: cover; /* Even if we have non-aspect ratio image */

  background-position: center center;     

}  

​

/* padding-bottom aspec-ration trick: */

​

.box {

  position: relative;

  height: 0;

  padding-bottom: 56.25%; /* Aspect ratio: 16:9 = 9 / 16 * 100 */

}

​

.box__in {    

  position: absolute;

  width: 100%;

  height: 100%;

}

​

/* etc: */

​

img {

  max-width: 100%;

  height: auto;

}

​

.container {

  max-width: 320px;

}