<img class="lazy" data-src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->resize(223); ?>" src=""  width="223" height="220" />

https://github.com/luis-almeida/unveil/blob/master/jquery.unveil.min.js

jQuery(document).ready(function() {
        jQuery('img.lazy').unveil();
});