The gallery option works like a sorth of featured image for the post. In the post I also added a gallery with other images then the 'postgallery' with one image used as featured image. I can not find a way to remove this gallery image from the post itself... Does anyone know?
Flashlight theme, featured image2 posts from 2 voices
Try switching to one of the default wordpress themes (like twenty eleven for example) and then go to that post. You should then be able to edit the featured image like normal.
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