query thumbs only posts

<ul>
    <?php $args = array(
    'cat' => 43,-5,
    'posts_per_page' => 4,
    'meta_query' => array(
        array(
            'key' => '_thumbnail_id',
        )
    )
 );
query_posts( $args ); ?>
            <?php while (have_posts()) : the_post(); ?>
                <li id="<?php the_ID(); ?>" class="pic">
                    <a href="<?php the_permalink(); ?>">
                        <?php echo the_post_thumbnail('thumbnail'); ?>
                    </a>
                </li>
            <?php endwhile; ?>
</ul>