div.video-holder {
  position: relative;
  padding-bottom: 56.25%; /* 315 / 560 (base of aspect ratio of video)*/
  height: 0;
}

iframe {
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
}

img {
  width: 100%; /* image resize according to viewport */
}