Add Images the_post_thumbnail_url()

<li class="professor-card__list-item">
  <a class="professor-card" href="<?php the_permalink(); ?>">
    <img src="<?php the_post_thumbnail_url(); ?>" alt="" class="professor-card__image">
    <span class="professor-card__name"><?php the_title(); ?></span>
  </a>
</li>