/* you have to open the portfolioPage.php if you are using the main portfolio page or the 
 porfolioGallery.php if you are using the portfolio gallery page and somewhere around the middle  of the file change this line: */

 <a rel="prettyPhoto[group]" class="single_image<?php echo($class); ?>" href="<?php echo($preview); ?>"></a>

/* to be like this: */

 <a class="single_image<?php echo($class); ?>" href="<?php 
echo(get_post_meta($post->ID, 'link', true)); ?>"></a>