your code looks correct, is it displaying the image?
I think you are just missing the anchor tag to make it a link:
<div id="lft_photo2_thumb"><a href="<path to full size image>/<?php echo $row_rs_tech_page['<full size image column']; ?>" rel="lightbox" ><img src="new_prod_thumb/<?php echo $row_rs_tech_page['prod_thumb2']; ?>" name="new_prod_thumb_2" width="120" height="80" border="0" id="new_prod_thumb_2" /></a></div>