adding a class to image links in posts

Is there a way to add a class to image links in posts? I am trying to open images with a lightbox while using default WP functionality (add media).

I am tying to go from

<a href="big-image.jpg">
  <img src="image.jpg">
</a>

to

<a class="lightbox" href="big-image.jpg">
  <img src="image.jpg">
</a>