I took this from the retina.js solution and modified it a bit. All credit goes to https://github.com/imulus/retinajs, I just tweaked this for my needs. This mixin includes most of the parameters you need, so you don't need to call them out twice. It can all be done in the mixin.

Less Retina Mixin Utility Class

// the retina mixin. An example goes: .at2x(img/header@2x.png, 650px, 150px, top, left, no-repeat, scroll)
// Put the size that the image appears on a non-retina, e.g. if it's a 200px x 200px non-retina, that is the effective size. The @2x 400px x 400px is calculated automatically. The order to type it is width then height.
// A modification of the retina.js less mixin - https://github.com/imulus/retinajs


@highdpi: ~"(-webkit-min-device-pixel-ratio: 1.5), (min--moz-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3/2), (min-resolution: 1.5dppx)";
.at2x(@path, @w: auto, @h: auto, @xpos: 0, @ypos: 0, @repeat: no-repeat, @attachment: scroll) {
  background-image: url(@path);
  background-position: @xpos @ypos;
  background-repeat: @repeat;
  background-attachment: @attachment;
  @at2x_path: ~`"@{path}".split('.').slice(0, "@{path}".split('.').length - 1).join(".") + "@2x" + "." + "@{path}".split('.')["@{path}".split('.').length - 1]`;
  @media @highdpi {
    background-image: url(@at2x_path);
    background-size: @w @h;
    background-position: @xpos @ypos;
    background-repeat: @repeat;
    background-attachment: @attachment;
  }
}

Less got stricter and I had some embarassing errors that used to work, but didn't, so they are now fixed.