How do I get old photos back in iOS 8 message
Not sure what your question is exactly, so I'm not sure I'm answering your problem.

Make sure that in Settings → Messages then scroll all the way down and Check the Message History preferences, the Audio Messages and Video Messages preferences, and after how long they're set to expire.

If you want to restore something into your messages, maybe try to Restore from an iCloud or a local iTunes backup if you have one.