Image doesn't show up using get_the_post_thumbnail_url();

Question

I have a simple question.

My wordpress theme has 2 pages: index.php and single.php. I've used the same code, but I had different results.

single.php

<?php

while(have_posts()) {
    the_post();
?>

    <h3><?php the_title(); ?></h3>
    Posted by <?php the_author(); ?>
    <img src="<?php echo get_the_post_thumbnail_url(get_the_ID()); ?>" />
    <?php the_content(); 

}

?>

In the code above, all the content shows up.

index.php

<?php

          $args = array(
            'posts_per_page' => 3,
          );

          $blogposts = new WP_Query($args);

          while($blogposts->have_posts()) {
            $blogposts->the_post();

        ?>

            <div class="col-lg-4">
              <div class="card" style="width: 18rem;">
                <img class="card-img-top" src="<?php echo get_the_post_thumbnail_url(get_the_ID()); ?>" alt="Img Post">
                <div class="card-body">
                  <h5 class="card-title"><?php the_title(); ?></h5>
                  <p class="card-text"><?php the_excerpt(); ?></p>
                  <a href="<?php the_permalink(); ?>" class="btn btn-primary">Leia mais</a>
                </div>
              </div>
            </div>

      <?php
        }
      ?>

In this case, all code is working, but the image didn't appeared.

What did I do wrong?

Answer 1

I've reached the "problem"

My theme had no "thumbnail" suport.

I just add the line add_theme_support( 'post-thumbnails' ); on funcions.php

Veja detalhes em https://codex.wordpress.org/Post_Thumbnails