The following registry patch will remove the background image from the Windows 10 sign-in screen. The download is a ZIP file, so you’ll need to extract it before you can merge the .reg file into your registry.  This file comes without warranty – use at your own risk.  If you want to disable the lock screen (the screen that displays before the sign-in screen), I’ve created a post about that as well.

Download: Win10_DisableLoginBackground.zip