Say you added an image in the text editor and want it to open in a lightbox popup.
In the editor make the image you placed there a link and link it to itself.
Then, paste the following class into the HREF tag (not the image tag).
So, your link will look something like this:
<a href="https://www.embryodesign.com/wp-content/uploads/2018/06/divi-tip.gif" class="et_pb_lightbox_image"><img src="https://www.embryodesign.com/wp-content/uploads/2018/06/divi-tip-150x150.gif" alt="" class="alignnone size-thumbnail wp-image-30280" width="150" height="150"></a>
Click the image below to see it work.