Nothing against NextGen Gallery, but migrating to Magnific Popup for Margaret Warren, I realized that the NGGallery files do not exist within the wordpress uploads folder, nor within the post database table.
Fortunately this neat little plugin migrates both the file and path info.
Thanks again, Konstantin Kovshenin.
Another great little tip on searching the wordpress forums for a specific theme’s discussions:
inurl:your_term “[theme:your_theme]” site:wordpress.org/support/topic