Downgrading from iOS 9.1 Beta to iOS 9 Stable and Keeping Picture Data
You can actually use your whole iOS 9.1 beta backup onto the iOS 9.0 stable release (installing backup made in newer firmware onto older firmware). I'm glad I managed it and I want to share the method.

First you have to backup in iTunes (while still in iOS 9.1 beta). Then, you have to find the directory where your backup is.


On a Mac, go to finder-->go-->go to folder, and input ~/Library/Application Support/MobileSync/Backup/
On Windows, go to \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\Windows


Find the latest folder (with lots of weird numbers and alphabets). You can sort by date to do this. Open the folder, and you'll get LOTS of text files. Search for info.plist and open it using a text editor or Xcode. Under "software version", it says 9.1. Change it to 9.0 and close the file.

You restore your device to iOS 9.0 in iTunes (since 9.0 is released it will restore it to 9.0 by default, not 8.4.1 anymore), and then after it's done you can restore from the backup you just made. It was made in iOS 9.1 but the computer thinks it's 9.0.