Working in tandem with create_deferred_img_tag.php, takes data from data-srcset and data-sizes of <img> elements and assigns them to the real srcset and sizes after the page has loaded lightweight images initially.

load-deferred-images.js

/**
     * Given a jQuery selector string corresponding to a collection of images, moves the values from the image attributes
     *  data-srcset and data-sizes to srcset and sizes, thus loading the appropriate images in a deferred manner.  Requires
     *  jQuery.
     * @param String selector A String used as a jQuery selector for a collection of images.
     * @param startIndex The start index of the subset of the images you want to load from the collection returned from the
     *  selector string.
     * @param endIndex The end index of the subset of the images you want to load from the collection returned from the
     *  selector string.  Put the same value for startIndex and endIndex if you only want a single image to be loaded.
     *  If not supplied, will be calculated as the last index in the collection.
     */
    function loadDeferredImages( selector, startIndex, endIndex ) {
        var $deferredImages = jQuery( selector );
        if ( endIndex == undefined ) {
            endIndex = $deferredImages.length - 1;
        }
        if ( $deferredImages.length > 0 ) {
            for ( var i = startIndex; i <= endIndex; i++ ) {
                if ( i < $deferredImages.length && i >= 0 ) {
                    if ( $deferredImages[ i ].getAttribute( 'data-srcset' ) ) {
                        $deferredImages[ i ].setAttribute( 'srcset', $deferredImages[ i ].getAttribute( 'data-srcset' ) );
                    }
                    if ( $deferredImages[ i ].getAttribute( 'data-sizes' ) ) {
                        $deferredImages[ i ].setAttribute( 'sizes', $deferredImages[ i ].getAttribute( 'data-sizes' ) );
                    }
                }
            }
        } else {
            return;
        }
    }