Problema con miniaturas en tema de WordPress para videos

<img src(unknown) ...

<?php get_header(); ?>
			<main>
			    <?php if ( have_posts() ) : ?>
				<?php echo mt_h2('<div class="ToroTitle fa-film">', '</div>'); ?>
				<ul class="ListPost AX Row Cols A06 B04 C03 D20 Sp15">
                    <?php while ( have_posts() ) : the_post(); ?>
					<!--<Post>-->
					<li>
						<article class="Post">
						    <?php
//if(function_exists(tubeace_activate)){ }
                            ?>
							<figure class="video-preview"><a href="<?php the_permalink(); ?>"><?php $thumb = tube_getcustomfield('wtp_thumb_url',get_the_ID());?>
							<img src="<?php echo $thumb; ?>" alt="<?php the_title_attribute(); ?>" title="<?php the_title_attribute(); ?>" /></a></figure>
							<header>
								<h2><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
								<div class="Info"><?php echo tr_categorias_post(get_the_ID(), 1); ?> <?php echo theme_duration_video(get_the_ID()); ?></div>
							</header>
						</article>
					</li>
					<!--<Post>-->
                    <?php endwhile; ?>
				</ul>
               
                <?php if(function_exists('wp_pagenavi')) { wp_pagenavi(); } ?>
                <?php
                else : 
                    get_template_part('inc/404');
                endif; 
                ?>
			</main>
<?php get_footer(); ?>

><?php $thumb = tube_getcustomfield('wtp_thumb_url',get_the_ID());?>