help me to display featured image of specific post by post id

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');

?>