bootstrap/scss/mixins/_image.scss

// Retina image
//
// Short retina mixin for setting background-image and -size.

@mixin img-retina($file-1x, $file-2x, $width-1x, $height-1x) {
  background-image: url("#{file-1x}");

  @media
  only screen and (-webkit-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-image: url("#{file-2x}");
    background-size: $width-1x $height-1x;
  }
}