Breaks on Windows 10 post 2016 anniversary update

[jpreece6]

• Pri file format has changed so the internal pri file editor no longer works to update the logon screen wallpaper.

[jpreece6]

After some investigation I've found that the background image is no longer managed by the Windows.Ui.Logon.pri file.

Storage Directory

As of the anniversary update logon backgrounds are now stored in: "C:\ProgramData\Microsoft\Windows\SystemData\"

Note you need to take ownership of the folder to access it.

Storage Format

System provided backgrounds are located in SystemData\S-1-5-18\ReadOnly\LockScreen_X Custom images selected via "browse" are located in SystemData\S-1-5-21......\ReadOnly\LockScreen_A

Windows names system backgrounds from Z upwards e.g. LockScreen_Z .... LockScreen_Y (Stops at ?) Custom images are named in the opposite fashion e.g. LockScreen_A .... LockScreen_B (Stops at G)

Data stored about custom images is stored in the registry at: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Lock Screen

Additional Info

Windows stores a copy of the original image named "LockScreen". Windows also modifies this image to match the resolution of the screen and the thumbnail size of the preview shown in settings. These modified copies are named to match their resolution e.g. "LockScreen_01080108" or "LockScreen1024_0768".

So far I've only noticed Windows to use the "LockScreen___1024_0768_notdimmed" version of these files on the lock screen.

Configuration

Windows uses the following key to determine which image to display on the lock screen:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\SystemProtectedUserData\S-1-5-21\LockScreen\Default

The "Default" key stores the letters of each image in order of display "ABCDEFG"... first letter is the letter used.

Note to modify the default key the owner needs to be changed to match the current user (admin) and add the user to permissions of that key.

Ideas

TODO