/**
 * The content
 * 
 * @param unknown $content
 * @return string
 */
function pronamic_photo_contest_the_content($content)
{
    if (is_singular('pronamic_photo_conte')) {
        $entries = pronamic_photo_contest_get_entries();
        $extra = '';
        if (empty($entries)) {
        } else {
            wp_enqueue_style('pronamic-photo-contest');
            $extra .= '<div class="pronamic-photo-entries">';
            $extra .= '<ul class="pronamic-photo-entries-list">';
            foreach ($entries as $entry) {
                $extra .= '<li>';
                $extra .= sprintf('<a href="%s">', get_permalink($entry->ID));
                $extra .= get_the_post_thumbnail($entry->ID, 'thumbnail');
                $extra .= sprintf('</a>');
                $extra .= '</li>';
            }
            $extra .= '</ul>';
            $extra .= '<div class="pronamic-photo-entries-clear"></div>';
            $extra .= '</div>';
        }
        $content .= $extra;
    }
    return $content;
}
<?php

global $post;
$entries = pronamic_photo_contest_get_entries();
wp_enqueue_style('pronamic-photo-contest');
?>
<div class="pronamic-photo-entries">

	<?php 
if (empty($entries)) {
    ?>
	
		<p>
			<?php 
    _e('No entries.', 'pronamic_photo_contest');
    ?>
		</p>
	
	<?php 
} else {
    ?>

		<ul class="pronamic-photo-entries-list">

			<?php 
    foreach ($entries as $entry) {
        ?>

				<li>					
					<?php 
        printf('<a href="%s" target="_blank">%s</a>', get_permalink($entry->ID), get_the_post_thumbnail($entry->ID, 'thumbnail'));