A simple css file, for adding support for CSS-classes that gives you the ability to serve high-resolution images.

highres.css

.hidden-normal-res {
  display: none;
}

@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  .hidden-high-res {
    display: none;
  }
  .hidden-normal-res {
    display: inherit;
  }
}

highres.html

<img class="hidden-high-res" style="width: 100px; height: 100px" src="100x100.png" />
<img class="hidden-normal-res" style="width: 100px; height: 100px" src="200x200.png" />