To add the lightbox to any image, when you are inserting (or go back and edit) a picture, on the screen where you can pick the size and add a caption, find the field: “Link to”.
Select “Media File” > Publish/Update and then the image will have a double border in your post and will will pop up the full size version in a lightbox. See example below.
If your image is already in the post and you want to add the lightbox, click the image and then the pencil icon to edit, and select as above.
