Windows 10 won't let me change my lock screen picture

You can select a new lock-screen image, but Windows returns to the default after restart, so the setting is not persisting. First confirm whether the PC is organization-managed; a policy can enforce a lock-screen background and will override Personalization settings. On a personal PC, use a local image stored in Pictures rather than a removable drive, choose it again in Settings, Personalization, Lock screen, and restart to test persistence. Install pending Windows updates and confirm that Windows is activated, because some personalization controls are restricted when activation is incomplete. If the image changes but reverts only after sign-in, test with a new local account; that distinguishes a profile problem from a system-wide configuration. Do not use registry cleaners or copied policy keys. If it persists on an unmanaged, activated device, capture the Windows version and the exact lock-screen behavior for Feedback Hub