mobile.css.scss

@mixin mobile_bg($file){
  background-image: image-url('mobile/standard/#{$file}');

  @media screen and (-webkit-min-device-pixel-ratio: 2) {
    background-image: image-url('mobile/retina/#{$file}');
    background-size: image-width('mobile/standard/#{$file}') image-height('mobile/standard/#{$file}');
  }
}

// Usage
.footer {
	@include mobile_bg('footer.png');
}