Смотрю код страницы firebug'om:
<div class="img_wrapper" style="display: inline-block; width: 234px; height: 309px;">
<img class="attachment-slider-post-thumbnail wp-post-image" width="310" height="407" title="slide11" alt="slide11" src="http://site.ru/wp-content/uploads/2011/07/slide11-310x407.jpg" style="position: absolute; z-index: 998; opacity: 1;">
<img class="attachment-slider-post-thumbnail wp-post-image" width="310" height="407" title="slide11" alt="slide11" src="data:image/png;base64,iVBORw0KGgoAAA..." style="position: absolute; opacity: 1;">
</div>