body {
  margin: 0;
  padding: 0;
  background-image: url('alcuk_small.jpg');
  background-repeat: repeat; /* tiled */
  background-size: auto;
  height: 100vh;
  width: 100vw;
}

/* Center the overlay image */
.overlay {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%); /* centers element */
}

.overlay img {
  max-width: 80%;
  height: auto;
  opacity: 0.9; /* optional transparency */
}