WordPress offers to define an image as a thumbnail for the complete post/page. My theme supports this, but currently no image is defined.
If I try to define this image wp ask me to upload an image, which I do not want to do. Instead I want to reuse an image of the gallery shown inside the page. Especially if the whole post is about the image gallery.
So how can I define an image uploaded with ngg as a post or page thumbnail?