Reference for image captions.

image_captions.html

<!-- Pre HTML5 -->
<div class="img-caption" itemscope itemtype="http://schema.org/ImageObject">
  <img src="" alt="" itemprop="contentURL">
  <span itemprop="description">
    Some image caption text.
  </span>
</div>

<!-- Post HTML5 -->
<figure class="img-caption" itemscope itemtype="http://schema.org/ImageObject">
  <img src="" alt="" itemprop="contentURL">
  <figcaption itemprop="description">
    Some image caption text.
  </figcaption>
</figure>