Small sass-mixin for the use of retina-images

example.sass

body
  background: url("bg.png") no-repeat
  @include image-2x("bg.png", 100px, 25px)

image-2x.sass

@mixin image-2x($image, $width, $height)
  @media (min--moz-device-pixel-ratio: 1.3), (-o-min-device-pixel-ratio: 2.6/2), (-webkit-min-device-pixel-ratio: 1.3), (min-device-pixel-ratio: 1.3), (min-resolution: 1.3dppx) 
    background-image: url($image)
    background-size: $width $height