Add filter to only load posts with thumbnails when the "show thumbnail" option is selected

upw-only-posts-with-thumbnail.php

add_filter('upw_wp_query_args', 'upw_only_posts_with_thumbnails',10 , 2);
function upw_only_posts_with_thumbnails($args, $instance) {

    if ( $instance['show_thumbnail'] ) {
        $args['meta_query'] = array(
            array(
                'key' => '_thumbnail_id',
                'compare' => 'EXISTS'
            )
        );
    }

    return $args;

}