<?php
$db = JFactory::getDbo();
$db->setQuery('SELECT m.file_url_thumb FROM #__virtuemart_product_medias pm,#__virtuemart_medias m WHERE (pm.virtuemart_product_id='.$item->virtuemart_product_id.')AND(pm.virtuemart_media_id=m.virtuemart_media_id)');
$res = $db->LoadResult();
echo "<img src=\"$res\">";
?>