select post preview image from nextgen gallery

Question

My website has a theme that uses the preview images in the post overview pages. However these images are neither automatically selected nor did I select an image in the past.

However, even if I try to select an image. For example within a page that only shows a gallery such as this one I can not select any image for the nextgen gallery. The only images that I see are the ones directly within /files.

Which were uploaded with the media library (which I dislike because it puts all files in a flat directory structure).

My gallery files are in the sub folder (and its subfolders) /files/gallery.

So how can I use gallery images for a post preview?