to link the portfolio image to the post rather than to the big imge open template_portfolio.php and search for 

echo '<a href="'.$portfolio_image.'" rel="lightbox[portfolio]"><img class="aligncenter" src="'.$portfolio_image_small.'" alt="" /></a>';

replace it with

echo '<a href="'.get_permalink().'"><img class="aligncenter" src="'.$portfolio_image_small.'" alt="" /></a>';