Hi res image mixin

2x-mixin.scss

div {
  @include image-2x("logo.png", "logo2x.png");
}

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