Sass Retina Mixin

@mixin retina-image($img, $img-hires) {
  background: url(/images/#{$img}) no-repeat center center !important;
  @media
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-device-pixel-ratio: 2),
  only screen and (   -moz-min-device-pixel-ratio: 2),
  only screen and (     -o-min-device-pixel-ratio: 2/1),
  only screen and (        min-device-pixel-ratio: 2),
  only screen and (                min-resolution: 192dpi),
  only screen and (                min-resolution: 2dppx) {
    background: url(/images/#{$img-hires}) no-repeat center center !important;
  }
}