body {
  background: url('//www.newhdwallpapers.in/wp-content/uploads/2014/03/Pier-Evening-Lights-HD-Wallpapers.jpg') no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size:    cover;
  -o-background-size:      cover;
  background-size:         cover;
}

body {
  background: url('//i.imgur.com/YahVxGw.jpg') no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size:    cover;
  -o-background-size:      cover;
  background-size:         cover;
}

Fix background not working on https
Fix background not working on https

newhdwallpapers.in doesn't work on https, sadly
