How to get_posts having post thumbnail?

The thumbnail is stored as a meta with a key of _thumbnail_id, so that could be used to find posts that have thumbnails. Something like this:

$query = new WP_Query(array(
    'meta_query' => array( array( 
        'key' => '_thumbnail_id',
        'value' => '0',
        'compare' => '>=',
        )
    )
));