Images 2x - retina images mixin

_mixin_2ximages.scss

@mixin image-2x($image1, $image2) {
  background-image: url($image);
  @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) {
    background-image: url($image2);
    background-size: image-width($image1) image-height($image1);
  }
}