The new version of Windows curiously doesn't allow you to change the image that displays behind the login screen when you first start your computer. This tiny utility, aptly called Windows 10 Login Background Changer, can do it for you.
The utility is available from Github. Once you download and run the application, you can choose a particular image or a solid colour to display on startup before you log in.
You can also choose to disable things like the user information or the networking and shutdown icons in the bottom-right of your screen, though it's unclear if this will break any functionality of your machine. Changing the background image, however, should work just fine.