Windows 10 defaults to images provided by MS randomly from the internet on the lock screen. I believe it's called Windows Spotlight and changes the lock screen image once per day. If your on Windows 10 Pro you can disable Spotlight via Group Policy. On Windows 10 Home you will have to disable it via the registry.
If you browse to HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\CloudContent, and create a new DWORD value called DisableWindowsSpotlightFeatures and set it's value to 1 it should disable it. However that setting is per-user, and if I remember right the lock screen has its own set of settings somewhere. But if you only have one user you might try this to start.