<?php
        $args = array( 'post_type' => 'main-slider' );
        $the_query = new WP_Query( $args );

        if($the_query->have_posts()) :
            while($the_query->have_posts()) : $the_query->the_post(); ?>
                <li><img class='img-full-width' src="<?php echo wp_get_attachment_url( get_post_thumbnail_id($post->ID), 'thumbnail' ); ?>" /></li>
            <?php endwhile;
        else :
            echo "<p>no hay contenido</p>";
        endif;
    ?>