Products w/o images

// WP_Query arguments
$args = array(
	'post_type'              => array( 'product' ),
	'order'                  => 'DESC',
	'orderby'                => 'id',
	'meta_query'             => array(
		array(
			'key'     => 'thumbnail_id',
			'value'   => '?',
			'compare' => 'NOT EXISTS',
		),
	),
);

// The Query
$query = new WP_Query( $args );