body,
html {
  height: 100%;
  margin: 0;
}

.bg {
  /* The image used */
  background-image: url('background.jpg');

  /* Full height */
  height: 100%;

  /* Center and scale the image nicely */
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}

@media screen and (max-width: 600px) {
  .bg {
    background-image: url('background-mobile.jpg');
  }
}