Windows 10 – Designing and setting a custom Lock Screen Background.

In short:

• it needs to be in a 16:9 ratio
• by default, the image is changeable by you in c:\Windows\System32\oobe\info\backgrounds\
  (Windows will copy that image and store it’s own copy under c:\ProgramData\… further details here)
• the Group Policy to manage this is Force a specific default lock screen image
• “On devices using other aspect ratios, such as 4:3 (1024×768) or 16:10 (1280×800), height scales correctly and width is cropped to a size equal to the aspect ratio. The image will remain centered on the screen”
• custom text is recommended to be stored within the 4:3 region of your custom screen, otherwise it will get cropped.

Reference:
Microsoft – Configure Windows Spotlight on the lock screen