Trick to remove is to delete the .thumbnails folder and create an empty .thumbnails file instead.
(http://devophuman.blogspot.com.au/2014/01/android-fills-flash-card-with.html)
Also, an empty .nomedia file could be used in specific folders not to be picked up by the gallery.
(http://smartadopter.wordpress.com/2010/05/06/tip-stop-unwanted-images-and-thumbnails-showing-in-the-android-photo-gallery/)
