Retina LESS Mixin v3 - now makes it super easy to specify one background image to cover three image sizes (@1x|@2x|@3x).

RetinaMixin3.less

.retina3(@bg-image, @size: cover,  @file-type: 'png'){
	background-image: url('@{path}@{bg-image}.@{file-type}');
	background-size: @size;

	@media only screen and (-webkit-min-device-pixel-ratio: 1.5),
		only screen and (min--moz-device-pixel-ratio: 1.5),
		only screen and (min-device-pixel-ratio: 1.5) {

			background-image: url('@{path}@{bg-image}@2x.@{file-type}');
			background-size: @size;
	}
	@media only screen and (-webkit-min-device-pixel-ratio: 3),
		only screen and (min--moz-device-pixel-ratio: 3),
		only screen and (min-device-pixel-ratio: 3) {

			background-image: url('@{path}@{bg-image}@3x.@{file-type}');
			background-size: @size;
	}
}