Bulma class: .is-overlay

<div class="card">
  <div class="card-image">
    <figure class="image is-4by3">
      <img src="https://bulma.io/images/placeholders/1280x960.png" alt="Placeholder image">
    </figure>
  </div>
  <div class="card-content is-overlay">
     <span class="tag is-primary">Image label on overlay</span>
  </div>
</div>