How does one detect when the wallpaper has changed (Windows XP or greater)?

I have figured out how to change the desktop wallpaper (there are dozens of examples on the Internet.)

One thing that still eludes me: how do I detect when the wallpaper has changed? (Say via the Display control panel or another program changing it.)

How do you change wallpaper when it's restricted?

Under Local Computer Policy, expand User Configuration, expand Administrative Templates, expand Desktop, and then click Active Desktop. Double-click Active Desktop Wallpaper. On the Setting tab, click Enabled, type the path to the desktop wallpaper that you want to use, and then click OK.

How do I change my wallpaper without activating Windows?

To set a different wallpaper on Windows 11 without activation, use these steps: Open Photos. Open the image to set it on the desktop. Right-click the image, select the Set as, and choose the Background option.

Add a message handler for WM_SETTINGCHANGE, SystemEvents.UserPreferenceChanged in .NET. Check if the wallpaper is still the same.