You should take the condition outide the loop meaning above
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
Therewise every time the loop begins it start the check instead of doing the check then starting the loop if the condition exist.
Its hard for to understand because i dont understand div names
but i think only should be inside the loop
<div id="contentholder">
<div class="uitleg">
<?php the_content(''); ?>
</div>
<div id="uitlegdivider"><img src="https://wordpress.stackexchange.com/questions/31814/<?php bloginfo("template_directory'); ?>/images/uitlegdivider.png" alt="" width="26" height="171" /></div>
<div class="voorbeeldimage">
<img src="<?php echo $drukproefimage; ?>" />
</div>
</div>
Explanation:
the loop = if (have_posts()) : while (have_posts())……
Anything iside that would go again and again until there are no posts fitting the condition you set before and not inside it.