A WordPress Post or Page can have a featured image. When the chosen image comes out an NextGEN gallery it will not show up in the header.
The problem and the solution are explained at the WordPress Support pages.
http://wordpress.org/support/topic/featured-image-not-working-with-nextgen-gallery
I’ve found a link that solves this very thing on almost all fronts – and it’s recent too:
http://lazytechguys.com/featured/fix-wordpress-featured-image-nextgen-gallery/