Regina ready background images, non-scss version...

retina-bg.css

element {
    background:url("path/to/image-1x") no-repeat;
}
@media (min--moz-device-pixel-ratio: 1.3), 
    (-o-min-device-pixel-ratio: 2.6 / 2),
    (-webkit-min-device-pixel-ratio: 1.3),
    (min-device-pixel-ratio: 1.3),
    (min-resolution: 1.3dppx) {
        element {
            background-image: url("path/to/image-2x");
            background-size: [width] [height];
            }
        }