/**
 * Identify images by class. For each image, add 
 * [Spin.js](http://fgnass.github.io/spin.js/) to parent, [LazyLoad](http://verlok.github.io/lazyload/) image, stop spinner 
 * when image is loaded.
 */

var imgClass = "lazy";

var spinOpts = {
  // [Spin.js options](http://fgnass.github.io/spin.js/#usage)
};

var spinners = [];
var elems = document.getElementsByClassName( imgClass );
for ( var i = 0; i < elems.length; i++ ) {
  var pId = "lazy-" + ( i + 1 );
  var parent = elems[i].parentElement;
  parent.id = pId;
  spinners[pId] = new Spinner( spinOpts ).spin( parent );
}

var lazyLoad = new LazyLoad( {
  elements_selector: "." + imgClass,
  callback_load: function( element ) {
    var spinner = spinners[element.parentElement.id];
    if ( spinner ) {
      spinner.stop();
    }
  }
} );

For use with LazyLoad by Andrea Verlicchi and Spin.js by Felix Gnass. SB and I have both checked out a few different lazyloading plugins, we’re pleased with how this one works with srcset.

Since the default positioning of Spin.js centres the spinner in the element, it’s best for the image to be the only child of the parent element. The code above assumes that this is the case. If I didn’t have control over the markup or needed to individually wrap each image for any other reason, would probably implement something similar to the above w/ jQuery (see the Spin.js jQuery plugin).

Wrote a function returning an image element with srcset and sizes attributes. See this Gist for the function and this Gist for an example of the function in use (would need to be within the WP loop).

Wanted to give a front-end dev like Sam the ability to define the important bits, including the default image size for the src attribute, the media queries for the sizes attribute, and classes for the img element if necessary.

Edit 23 Jan 2019
This isn’t necessary anymore, responsive images have been part of WP core since v4.4. They’re implemented on wp_content automatically via a filter. Use their related functions if you need to do something custom. Responsive images are behaving a little erratically on my site though, so will have to take a look at why that might be happening.