WordPress blog archive: featured image or dynamic random featured image

after.php

<div class="list-article-thumb">
    <a href="<?php echo esc_url( get_permalink() ); ?>">
        <?php
            if ( has_post_thumbnail( ) ) {
                the_post_thumbnail();
            } else {
                $images_iterator = new FilesystemIterator(
                    WP_CONTENT_DIR . '/themes/yourthemename/images/',
                    FilesystemIterator::SKIP_DOTS
                );
        
                $rand = rand(1, iterator_count($images_iterator));

                echo '<img alt="" src="'. get_stylesheet_directory_uri() . '/images/' . $rand . '.jpg' .'">';
            }
        ?>
    </a>
</div>