retina mixin

@mixin image-2x($image, $width, $height) {
  @media (min--moz-device-pixel-ratio: 1.3),
         (-o-min-device-pixel-ratio: 2.6/2),
         (-webkit-min-device-pixel-ratio: 1.3),
         (min-device-pixel-ratio: 1.3),
         (min-resolution: 1.3dppx) {
    /* on retina, use image that's scaled by 2 */
    background-image: url($image);
    background-size: $width $height;
  }
}

<!-- then use -->
div.logo {
  background: url("logo.png") no-repeat;
  @include image-2x("logo2x.png", 100px, 25px);
}