Php code help ? Can't find the problem

Alright i’m trying to get some ”next picture” previous picture” buttons on my website. ( http://www.memesquebecois.com )When i try to change picture it goes to a half loaded page (see by yourself, just go on my website and click the arrows)

Here is the code:

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<H1><?php the_title(); ?></H1>
<div class="clear"></div>
<div class="buttons">

<?php previous_post_link_custom('%link', 'Previous Pic') ?>
<?php next_post_link_custom('%link', 'Next Pic') ?>
</div>
<div class="pic rounded-8">

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

[please mark any posted code – http://codex.wordpress.org/Forum_Welcome#Posting_Code ]

where is the problem ?!