retina.js
JavaScript helpers for rendering high-resolution image variants
...For example, if you have prepared 3x variants, retina.js will assume that you have also prepared 2x variants. If the environment does have 3x capabilities, retina.js will serve up the 3x image. It will expect that url to be /images/my_image@3x.png. If the environment has the ability to display images at higher densities than 3x, retina.js will serve up the image of the highest resolution that you've provided, in this case 3x.