Mixin: Retina

@mixin retina {
	@media screen and (-webkit-min-device-pixel-ratio: 1.5),
	screen and (-moz-min-device-pixel-ratio: 1.5),
	screen and (-o-min-device-pixel-ratio: 3 / 2),
	screen and (min-device-pixel-ratio: 1.5),
	screen and (min-resolution: 1.5dppx) {
		@content;
	}
}

// Usage

.element {
	@include retina {
		background-image: url(../img/background@2x.png);
	}
}