set custom gallery image

If the gallery is viewed in teaser mode, the first image of the gallery is taken to represent the content of the gallery. If you like to have another image in the teaser-view, you need to move the specific media to the first place in your gallery.

I think, it would be great to add a new optional (image-)field, which represents the content of the gallery. If the field is empty, we can fill it with the first image - as we do it now.

This would make the media_gallery_media_original property obsolete.