this is the function:
# Displays post image attachment (sizes: thumbnail, medium, full)
function dp_attachment_image($postid=0, $size='thumbnail', $attributes='') {
$the_query = new WP_Query(array('p'=>$postid));
$the_query->the_post();
the_post_thumbnail($size);
wp_reset_postdata();
}
which is basically a wrapper for 'the_post_thumbnail()'
do your posts have the 'featured image' ?