dual-bg-image.css

div {
  background-image: url('/images/background.png');
  @media only screen and (-webkit-min-device-pixel-ratio: 2), only screen and (min-device-pixel-ratio: 2) {
    background-image: url('/images/hi-background.png');
  }
}

dual-bg-image.scss

/*
  Mixin for creating both standard background image
  and hi-res iPhone 4 background image
*/
@mixin dual-bg-image($image) {
  background-image: image_url($image);
  // media query from: http://miniapps.co.uk/blog/post/targeting-iphone-4-using-css-media-queries/
  @media only screen and (-webkit-min-device-pixel-ratio: 2), only screen and (min-device-pixel-ratio: 2) {
    background-image: image_url('hi-'+$image);
  }
}

div {
  @include dual-bg-image('background.png');
}