This plugin hasnt been tested with the latest 3 major releases of WordPress. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.
Just to add some details, the plugin did not work after I installed and placed the image as described in the instruction.
However, when I added the keyword lightbox in the "Link Rel" under "Image Details > Advanced Options" the lightbox started to work.
I had previous experience working with lightbox so hope this helps if someone ran into the same problem as I did.