Understanding Retina device CSS Media queries - image

Question

I am working on a WordPress theme and am trying to incorporate retina enabled CSS queries into my CSS file.
I would just like to clarify that I have the media queries set up correctly before I change out all my background images.
I have doubled the size of all my background images and perfixed
them with the "#2x" naming convention. e.g icon-user#2x.png.
I have added a jQuery function into my code to swap out the images with the CSS class of hires.
In my CSS document I have a normal CSS class for a background image.
Normal CSS query
.side-nav .arrow {
background: url(../images/arrow-nav.png) no-repeat top left;
width: 5px;
height: 8px;
display: inline-block;
margin-left: 10px
}
Is this the correct way i would change the .side-nav .arrow class for a retina enabled device? When declaring the background size do I keep the size that of the original smaller image?
/* All Retina Ready devices larger than 1.5 pixel ratio */
#media only screen and (-moz-min-device-pixel-ratio: 1.5),
only screen and (-o-min-device-pixel-ratio: 3/2),
only screen and (-webkit-min-device-pixel-ratio: 1.5),
only screen and (min-device-pixel-ratio: 1.5) {
.side-nav .arrow {
background-image:url(../images/arrow-nav#2x.png);
-webkit-background-size:5px 8px;
-moz-background-size:5px 8px;
-o-background-size:5px 8px;
background-size:5px 8px
}
}
jQuery Code
$(function () {
if (window.devicePixelRatio == 2) {
var images = $("img.hires");
/* loop through the images and make them hi-res */
for(var i = 0; i < images.length; i++) {
/* create new image name */
var imageType = images[i].src.substr(-4);
var imageName = images[i].src.substr(0, images[i].src.length - 4);
imageName += "#2x" + imageType;
/* rename image */
images[i].src = imageName;
}
}
});
Thank you

Answer 1

As long as there is some form of scaling taking place, like when you declare
<meta name="viewport" content="width=..."> (for android/ios/blackberry/WP8)
or
#ms-viewport {width: ... ;} (for non-WP8 IE10)
or ... even if you declare nothing most mobile devices will by default automatically scale such that viewport width=980px
then all CSS dimensions you declare with 'px' will exist in the same proportion to their viewport regardless of differences between their physical DPI/PPI
this means you shouldn't have to change a single thing about your style class except the background image's URL when the media query matches a high res device:
#media only screen and (-moz-min-device-pixel-ratio: 1.5),
only screen and (-o-min-device-pixel-ratio: 3/2),
only screen and (-webkit-min-device-pixel-ratio: 1.5),
only screen and (min-device-pixel-ratio: 1.5),
only screen and (min-resolution: 144dpi) {
.side-nav .arrow {
background-image:url(../images/arrow-nav#2x.png);
}
}