gistfile1.txt

@mixin retina-image($filename, $image-width, $image-height, $extension: ".png") {
    background-image: url($filename + $extension);
    height: $image-height;
    width: $image-width;

    @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($filename + "2x" + $extension);
        background-size: $image-width $image-height;
    }
}​