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#11
OK, got it.
You have to edit the registry.
1. Open the registry editor through your preferred method.
2. In the Registry Editor, navigate to HKEY_LOCAL_MACHINE > Software > Policies > Microsoft > Windows > System.
3. Click on System and look for an entry that says DisableLogonBackgroundImage. If you have the entry, change the value from 1 (True) to 0 (False). Lock your screen and test. This is what worked for me, as I had the entry.
3a. If you don't have that entry, create it. Right click (in the right pane, where the entries go) and select New > DWORD (32-bit) Value. Label this new DWORD DisableLogonBackgroundImage.
4. Right-click on DisableLogonBackgroundImage and click Modify (or just double click on the entry name).
5. Under Value data, make sure the value is 0 type 1 (default will be 0). Click OK. (To switch back to the default Windows log-in screen, change this value to 1.)
This is the article where I got this, except 0 enables my image and 1 disables choosing it on my machine; that is, the article has it backwards.
Change the Windows 10 log-in screen background - CNET
[Edit: Oops... didn't see the second page. You beat me to the solution!]