Use an if statement around the image tag to only show it if there is a value in the recordset for that column, for example:
<?php if($row_recordsetname["prod_image_large"] != "") { ?><a href="new_prod_image_large/" rel="lightbox"><img src="new_prod_thumb/" alt="" name="new_prod_thumb_2" width="120" height="80" border="0" id="new_prod_thumb_2" /></a><?php } ?>