Delete all image, meta data and post permanently.
# delete all posts
$args = array(
'post_type' => 'post',
'posts_per_page' => -1,
'post_status' => 'any'
);
$posts = get_posts($args);
foreach ($posts as $post) {
# delete post thumbnail
$thumbnail_id = get_post_thumbnail_id($post->ID);
if ($thumbnail_id) wp_delete_attachment($thumbnail_id, true);
wp_delete_post($post->ID, true);
}