In a ScreenSteps document, when an image is inserted the HTML looks like this:
<div class="image"> <a class="screensteps-image-zoom" href="https://s3.amazonaws.com/screensteps_live/images/original.png"> <img src="https://s3.amazonaws.com/screensteps_live/images/display.png" alt="my image"> </a> </div>
When adding your lightbox library you will want to reference the linked image. For example:
Using that selector, you should be able to integrate your favorite lightbox library into your Zendesk Help Center.