On the Guilford Church site, I noticed that the icon to close the lightbox window was missing. The Web Developer’s Toolbar showed me that that and the “loading” image were trying to load from a content item/plugin/….path.
I had to open up the lightbox.js and add ../../ to the front of the image path.
The problem was that the pages with the Simple Image Galleries were at various levels in the site, (with Joomla! built in SEF).