If you use an email address and password to sign in, you're using a Microsoft account. Change your account picture in Windows Select the Start > Settings >. Go to Settings > Personalization > Lock screen. Under Background, select Picture or Slideshow to use your own picture(s) as the background for your lock.      

How to Change the Login Screen Background on Windows 10 « Windows Tips :: Gadget Hacks.

  If you use an email address and password to sign in, you're using a Microsoft account. Change your account picture in Windows Select the Start > Settings >. Head to Settings > Personalization > Lock screen and enable the “Show lock screen background picture on the sign-in screen” option here. You can. how to change the desktop and login screen image in windows 10 select picture for background. Step 2: Click the drop-down menu underneath.