The “the_post_thumbnail()” function for featured image in WordPress does not shows anything if a featured image is not attached to the post. In case you want to show a default image in your theme if a user has not attached any featured image then use following script:
<a href="<?php the_permalink(); ?>"> // adds a hyperlink to the image <?php if ( has_post_thumbnail() ) { //check if featured image is added or not the_post_thumbnail( 'medium', array( 'class' => 'img-responsive' ) ); //echos featured image if added } else { ?> <img src="<?php echo get_template_directory_uri(); ?>/images/defaultimg.png" class="img-responsive" alt=""> // outputs default image if not . change the url to match your image. <?php } ?> </a>