There was a Messages bug back in iOS 6.x.x where if you deleted a message, the attached photo or video was left "hung" in your messages threads and was no longer accessible because the message it was tied to was gone. I believe that bug in the deletion was fixed in iOS 7, but the old "orphaned" media is still in your messages database on your device. From what I understand, the only way to clear it is to reset your phone as a new device and set it up from scratch.
Some users have reported ways to go into the iPhone backups and delete the images, then to restore from that backup, but it is not something that I have done, and I'm not certain I would recommend it. You may find a third party app for your computer which will allow you to access your messages and delete the attachments from there if you don't ant to restore as new and start over...