source: extensions/Versa/js/thumbnails.loader.js

if ( typeof( max_requests ) == "undefined" )
  max_requests = 3;

var thumbnails_queue = jQuery.manageAjax.create('queued', {
  queue: true,  
  cacheResponse: false,
  maxRequests: max_requests,
  preventDoubleRequests: false
});

function add_thumbnail_to_queue(img, loop) {
  thumbnails_queue.add({
    type: 'GET', 
    url: img.data('src'), 
    data: { ajaxload: 'true' },
    dataType: 'json',
    beforeSend: function(){jQuery('.loader').show()},
    success: function(result) {
      img.attr('src', result.url);
      img.parent().css( "background-image", "url(" + result.url + ")" );
      img.parent().css( "background-size", "" );
      jQuery('.loader').hide();
    },
    error: function() {
      if (loop < 3)
        add_thumbnail_to_queue(img, ++loop); // Retry 3 times
      if ( typeof( error_icon ) != "undefined" )
        img.attr('src', error_icon);
      jQuery('.loader').hide();
    }
  }); 
}

function pwg_ajax_thumbnails_loader() {
  jQuery('img[data-src]').each(function() {
    add_thumbnail_to_queue(jQuery(this), 0);
  });
}

jQuery(document).ready(pwg_ajax_thumbnails_loader);