You can only delete photos from the Camera, Saved, or Imported rolls/albums. Pictures that were synced through iTunes can not be removed except by changing your iTunes sync settings, or editing the synced folder on the computer and re-syncing.
Photos in albums you create on the iPad can be deleted only if they were added from one of the same three editable albums. If they were added from a synced album they can only be removed from your created album.
The PhotoStream can not be deleted. You can only reset the entire stream (removing all pictures) through the icloud.com site; ; which does not work in Safari on the iPad.