gistfile1.scss

@mixin at2x($image_name, $width, $height, $extention: '.png') {
  background-image: image_url($image_name + $extention);
  $x2img : $image_name + '@2x' + $extention;

  @media (-webkit-min-device-pixel-ratio: 1.5),
         (min--moz-device-pixel-ratio: 1.5),
         (-o-min-device-pixel-ratio: 3/2),
         (min-device-pixel-ratio: 1.5),
         (min-resolution: 1.5dppx) {
    background-image: image_url($x2img);
    background-size: $width $height;
  }
}

.logo {
  @include at2x('logo', 80px, 30px);
}