Retina background image mixin for LESS

retina.less

// Set the background image for retina devices
.background-image-retina(@url, @ext, @width, @height) {
  @lowres:  "@{url}.@{ext}";
  @highres: "@{url}@2x.@{ext}";

  background-image: url(@lowres);
  background-size: @width @height;
  
  @media 
  only screen and (-webkit-min-device-pixel-ratio: 2),
  only screen and (   min--moz-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-image: url(@highres);
  }
}