How to completely erase the Photo Library on an iPhone? I decided to delete all of my iPhone's photos and videos so I copied everything to iPhoto and deleted everything on my phone. After that, I decided to backup my phone through iTunes. Then I reinstalled iOS 7.1 through iTunes (via ipsw and Restore phone). After a few minutes, I was able to restore everything. The phone is much faster than before. The only problem is, the Photo Library still consumes space.
Photos and videos shared through MMS or iMessage take up space. You can delete these to reclaim space from Messages.

Source: http://forums.everythingicafe.com/threads/iphone-camera-roll-empty-but-storage-2gig.105115/#post-1047178