html-snippet.html

<img src="images/header.png" data-srcorig="images/[email protected]">

retinaify.js

// retinaify imgs
var isRetina = window.devicePixelRatio > 1;
if (isRetina) {
  var imgs = document.getElementsByTagName('img');
  for (var i = 0; i < imgs.length; i++) {
    var img = imgs[i];
    var src2x = img.dataset && img.dataset.src2x;
    if (src2x) {
      img.setAttribute('data-src-orig', img.src);
      img.src = src2x;
    }
  };
}