Have a look at the documentation for retina.js on GitHub. Scroll down to the section that describes SASS, SCSS and LESS. Below those you'll see how the CSS is rendered. You didn't mention whether or not you're using any of the preprocessors, but in case you are, I am quoting that site below:
SCSS
#item {
@include retina('/images/my_image.png', 3, cover, center center no-repeat);
}
Sass
#item
+retina('/images/my_image.png', 3, cover, center center no-repeat)
Less
#item {
.retina('/images/my_image.png', 3, cover, center center no-repeat);
}
Stylus
#item
retina('/images/my_image.png', 3, cover, center center no-repeat)
Regardless of the dialect, the output is effectively the same:
#item {
background: url("/images/my_image.png") center center no-repeat;
background-size: cover;
}
@media all and (-webkit-min-device-pixel-ratio: 1.5),
all and (-o-min-device-pixel-ratio: 3 / 2),
all and (min--moz-device-pixel-ratio: 1.5),
all and (min-device-pixel-ratio: 1.5) {
#item {
background: url("/images/my_image@2x.png") center center no-repeat;
background-size: cover;
}
}
@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
#item {
background: url("/images/my_image@2x.png") center center no-repeat;
background-size: cover;
}
}
@media (-webkit-min-device-pixel-ratio: 3), (min-resolution: 288dpi) {
#item {
background: url("/images/my_image@3x.png") center center no-repeat;
background-size: cover;
}
}