Sometimes I have to explain this logic, this gist makes it easier. ;)

Upgrading images smoothly

// Quick implementation of ResponsiveEnhance by Josh Emerson
// Original implementation: https://github.com/joshje/Responsive-Enhance


// <img src="sd.jpg" data-hd="hd.jpg">
function quickUpgrade (obj) {

    var tmp = new Image();                      // prepare a loose image to make the request & cache it
    var newsrc = obj.getAttribute('data-hd');   // grab the URL of hd version
    if (newsrc) {                               // make sure it's there

        tmp.onload = function () {              // do this only when new img is loaded & cached
            obj.src = newsrc;                   // switcheroo
        };

        tmp.src = newsrc;                       // start the request (while leaving sd version visibile/in DOM)

    }
}