add_action( 'save_post', 'save_default_thumbnail' );
function save_default_thumbnail( $post_id ) {
$post_thumbnail= get_post_meta( $post_id, '_thumbnail_id', true );
if ( !wp_is_post_revision( $post_id ) ) {
if ( empty( $post_thumbnail ) ) {
update_post_meta( $post_id, $meta_key= '_thumbnail_id', $meta_value= 'アッタチメントIDを記述' );
}
}
}