Your answer was almost perfect and certainly led me to the right path.
The Lightbox plugin to enable the Lightbox effect in Gallery NextGen is actually not recommended and is replaced by the Slimbox plugin by my compatriot Giuseppe Argento.
The official WordPress page for the plugin is here:
http://wordpress.org/extend/plugins/slimbox-plugin/
Just install the plugin, then click on the link in the gallery settings:
Gallery β Options β Effects
Now you can select βLightboxβ and it will work perfectly.
Just make sure your images are not too large, as Lightbox / Slimbox does not resize the images that appear in the modal layer above your web page.
I usually shoot a maximum size of 800 x 600.
Mike
source share