Retina images feature serves different images based on the device being used by the viewer. You can find many details here on how it works http://retina-images.complexcompulsions.com/
Currently in the website, the php file exist under the name of _retinaimages.php in the php-helpers folder. Also i’ve added in
Why the code is commented?
Well , i’m not a big fan of retina images, although it looks like it makes the web a better place, the cost of bandwidth is way too big. Ironically most smartphone devices with limited bandwidth have Retina or better said doubled
devicePixelRatio . The cost is too big and perhaps in a few years with 4G+ speeds and perhaps unlimited bandwidth we can truly forget about website sizes, for the moment i recommend the tool only where you really need to use it.