You can easily set any image you like as your login screen background on Windows 10. This was complicated in the initial release of Windows 10, but Microsoft made it simple with the Anniversary Update.
Head to Settings > Personalization > Lock screen and enable the “Show lock screen background picture on the sign-in screen” option here.
You can configure the sign-in screen background you want on the Lock screen settings page, too. Select “Windows spotlight” for an automatically changing selection of images from Microsoft, or select “Picture” or “Slideshow” to choose your own preferred background image.
The image you select here will appear as both your lock screen background and sign-in screen background.
If you’d rather use a flat color instead of a background image, disable the “Show lock screen background picture on the sign-in screen”. Navigate to Settings > Personalization > Colors. The color you choose here will be used for your sign-in screen background as well as other elements on the Windows desktop.