Windows 10 included Hero wallpaper that comes as a default wallpaper in final build of Windows 10. It also popped in the same image for the login background.
It might have pleased a few people but most of the people out there don’t seem to have liked it.
DISABLING THE LOGIN SCREEN BACKGROUND IMAGE IN WINDOWS 10
Go to Start > type regedit
On the folder tree (left side of regedit window), open HKEY_LOCAL_MACHINE > SOFTWARE > Policies > Microsoft > Windows > System
By clicking System, you can see (Default) file on the right pane of the window.
Right-click on the empty area > New > DWORD (32 bit) Value
Name the file DisableLogonBackgroundImage (Remember: Name the file exactly mentioned here)
Double-click the file:
To remove the Windows Hero background, set the Value data to 1.
To restore the Windows Hero background, set the Value data to 0.
This method sets the accent color to the background image in login screen. In the internet, you may find 3rd party software that commits to set user defined image but after the KB3081424 Windows Update, it seems to cause screen flickering issue between Lock Screen and Login Screen.