bulma/docs/_includes/book/book-cover.html

<div class="bd-book-cover">
  <a href="{{ site.data.meta.book_url }}" target="_blank">
    {%
      include elements/responsive-image-2x.html
      path="book/book-cover"
      extension="jpg"
      alt="The official Bulma book cover"
      width="504"
      height="661"
    %}
  </a>
</div>