// Retina image
.retina-image(@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 (   min--moz-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;
    }
}