<?php  if ( has_post_thumbnail() ) : the_post_thumbnail();?>
 <?php   echo'<img class="img-fluid d-block" src="<?php the_post_thumbnail_url(); ?>">'; ?>
<?php elseif (): ?>
 <?php echo'<div class="embed-responsive embed-responsive-4by3">
      <iframe class="embed-responsive-item" src="<?php echo get_post_meta($post->ID, 'video', true); ?>" allowfullscreen=""></iframe>
    </div>';?>
<?php else (): ?>
  <?php  echo'<img src="<?php bloginfo('template_directory'); ?>/screenshot.png"  width="64" height="64" alt="<?php the_title(); ?>" />';?>