I am using the caption template to display a gallery, but I noticed that the captions did not show, I did a vardump of an image and it did not show content for the caption as it is set in the gallery.
I noticed that the alttext had the same content as the caption so I changed the code. to print the alttext in setad of the caption.
Then I noticed that my Lightbox Plus popup displayed the description value of the image in the gallery instead of the caption.
for an example see http://www.kuilder.net/a-quick-test-of-the-nextgen-gallery/