You can use get_the_post_thumbnail like this:
<?php $mypost = get_post(28); echo apply_filters('the_content',$mypost->post_content); ?>
<?php $mypost = get_post(28);
echo apply_filters('website',$mypost->website);
echo apply_filters('titleback',$mypost->titleback);
echo apply_filters('contentback',$mypost->contentback);
echo get_the_post_thumbnail( $mypost->ID, 'thumbnail');
?>