html, body {
  margin: 0;
  padding: 0;
  background-color: #000;
  height: 100%;
  overflow: auto;
}

.fullscreen-container {
  width: 100%;
  text-align: center; /* horizontal centering */
}

.fullscreen-container img {
  width: 100%;
  height: auto; /* scale proportionally */
  display: block;
}