Попробуйте это в шаблоне single.php:
$args = array(
'post_type' => 'attachment',
'post_mime_type' => 'image',
'post_parent' => $post->ID
);
$images = get_posts( $args );
foreach($images as $image):
echo wp_get_attachment_image($image->ID, 'medium');
endforeach;