To create a lightbox-style image in a page or post, click the green cross short code symbol in the editor, and select Lightbox. Enter the dimensions for the thumbnail preview, and paste the image URL that you want (probably the easy way to do this is have another window open with the media library – copy the File URL of the image you want to use and paste it into the image URL for the lightbox embed). Then click Insert Shortcode to insert the image into the page.