test.css

@media (max-width: 767px) {
  .box-slide.bg-5 {
    background-image:url(http://i.imgur.com/OFx35dP.png);
    background-size: 100% 340px;
  }
  .box-slide.bg-4 {
    background-image:url(http://i.imgur.com/ckNliyV.png);
    background-size: 100% 340px;
  }
}

@media (max-width: 600px) {
  .box-slide.bg-5 {
    background-image:url(http://i.imgur.com/yJMdhYi.png);
    background-size: 100% 340px;
  }
  .box-slide.bg-4 {
    background-image:url(http://i.imgur.com/TWuerXa.png);
    background-size: 100% 340px;
  }
}