Beitragsbild als Hintergrund

  <div class="card-body"
    <?php if(has_post_thumbnail()): ?>
      style="background-image: url('<?php the_post_thumbnail_url();?>');"
    <?php endif;?>
  >

Quelle: https://stackoverflow.com/questions/43576107/how-to-set-a-post-featured-image-as-background-image-in-wordpress