html {background-image: url(https://images.pexels.com/photos/1591447/pexels-photo-1591447.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750&dpr=2);
  background-size: cover;
  background-position: center;
background-attachment: fixed}

Background-size cover makes the image cover the whole screen
Background-position center centers the image
Background-attachement fixed makes it so the image doesn’t move when you scroll down the page.