Confused with the loop and repeating things

You are not using loop properly. Not tested but you can try this:

<?php if (have_posts()) :  ?>
    <br />
    <div class="jumbotron">
        <?php while (have_posts()) : the_post(); ?>

            <?php the_post_thumbnail(array('class' => 'img-thumbnail')); ?>

        <?php endwhile; ?>

        <br /><br />
        <button type="button" class="btn btn-xs btn-warning">VER TODOS LOS ESTRENOS</button>
    </div>

<?php else: ?>

<p><?php _e('No posts were found. Sorry!'); ?></p>

<?php endif; ?>