When adding an image into a content body, there is a new option to "Include Full-Resolution Popup". When "yes" is selected the image will be inserted with code to enable the full-resolution popup. When "no" is selected the image will be inserted same as always.

Note "yes" should only be selected if you've uploaded a high-resolution image.
Note the popup will not work if the image contains a link, as the link will take the user to the link's destination.

Here is where you can find the option to add a full-resolution lightbox when adding an image to your article

Here's an example of how the lightbox will appear when activated within the article.