how to handle high res display.

bgImgForRetina.css

/* load http://www.modernizr.com/ then, do following for Retina display. */

.className {
    background:url(test@1x.png); /* low res img */
}
@media only screen and (-webkit-min-device-pixel-ratio:2) {
    .backgroundsize .className {
        background:url(test@2x.png); /* high res img */
    }
}