How to use thumbnails with next_post_link?

Hi,

I want to use the thumbnails of posts with next_post_link & previous_post_link instead of the basic hyper link.

This is the current code of my Previous & Next post :

<div class="alignleft">
<?php next_post_link('&laquo; %link') ?>
</div>
<div class="alignright">
<?php previous_post_link('%link  &raquo;') ?>
</div>
<div style="clear: both"></div>

And this is the code of thumbnail links as used on the category & index pages :

<!–post title as a link–>
    <div id="post-<?php the_ID(); ?>" class="thumbnail" >
    <?php $feature_img = portfolio_get_post_image( $post->ID, 'Thumbnail', 256, 125 ); ?>
    <a href="<?php echo get_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>">
    <img src="<?php echo $feature_img; ?>" alt="<?php the_title(); ?>" /></a>
    </div>

Thank you so much for your help.