[video] YouTube embedded video: set different thumbnail

This solution is HTML and CSS based using z-index and hover, which works if JS is disabled or the video isn't yours (since you can add a thumbnail in YouTube).

<style>
.videoWrapper {
  position: relative;
  padding-bottom: 56.25%;
}
.videoWrapper .video-modal-poster img {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: 10;
}
.videoWrapper .video-modal-poster:hover img {
  z-index:0;
}
.videoWrapper iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: 1;
}
</style>
<div class="videoWrapper">
  <a href="#" class="video-modal-poster">
    <img alt="" src="" width="353" height="199" />
    <iframe width="353" height="199" src="https://www.youtube.com/embed/" frameborder="0" allowfullscreen="allowfullscreen"></iframe>
  </a>
</div>