By default, Windows 10 shows background pictures on your lock screen that have been curated specifically for this use–but it’s not immediately clear where they’re stored. Windows replaces these images regularly, but if you want to use them as regular wallpapers, the last several are usually in that cache and are not too hard to save if you grab them in time.
Many of you may have disabled Spotlight images on your lock screens because Microsoft will slip the occasional ad in there, but if you haven’t, you’ll notice that the ads are pretty rare, and the Spotlight images are often really nice. Also note that we’re talking here about background images for the lock screen–that page you have to click or slide out of the way to get to the login screen. You can actually set background images for your login screen separately.