NextGen is looking for photos in
But /wp-content/gallery/ does not exist
All of my photos are in /wp-content/uploads/yyyy/mm
In the wp administrator, when I go to Manage Gallery, none of the thumbnails show, for the same reason.
Here's an example problem page: http://putneyhistory.us/photos/
On a newer site I created I see that the photos are all in /wp-content/gallery/gallery-name, so I am guessing that this is a change that happened during a NextGen upgrade.