you need to be comfortable with using wordpress custom fields. you need to create a new custom field, maybe call it "new_link" and then enter the link into this custom field. then you need to edit the template files.


<h3 id="post-<?php the_ID(); ?>"><a href="<?php echo get_permalink() ?>" rel="bookmark" title="Permanent Link: <?php the_title(); ?>"><?php the_title(); ?></a></h3>
<?php echo '<a href="'.$portfolio_image.'" rel="lightbox[portfolio]"><img class="aligncenter" src="'.$portfolio_image_small.'" alt="" /></a>';

change this to:  

$link = get_post_meta($post->ID, "new_link", true);
<h3 id="post-<?php the_ID(); ?>"><a href="<?php echo $link; ?>" rel="bookmark" title="Permanent Link: <?php the_title(); ?>"><?php the_title(); ?></a></h3>
<?php echo '<a href="'.$link.'" ><img class="aligncenter" src="'.$portfolio_image_small.'" alt="" /></a>';