gistfile1.scss

@mixin background-image-retina($file, $type, $width, $background-repeat: no-repeat) {
  background-image: image-url($file + '.' + $type);
  width: $width;
  background-repeat: $background-repeat;

  @media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 144dpi) {
    & {
      background-image: image-url($file + '@2x.' + $type);
      background-size: $width;
    }
  }
}