If its a factory installed wallpaper you cant retrieve it they are built into the software but if its a wallpaper you added it would be in your Gallery
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If you use Good Lock's LockStar plug-in, you can remove clock, widgets, notifications, etc from the lock screen and then take a screenshot of it. The screenshot will end up in your Gallery and can then be used for wallpaper.
If your device is rooted, you can reach the current wallpapers in the paths:
"/data/system/users/0/wallpaper_orig" for the home wallpaper
"/data/system/users/0/wallpaper_lock_orig" for the lock screen
They don't have an extension, but they should be image files.
If you don't have a rooted device, you have 2 options:
Backup+ - A backup app of various things. It fetches the wallpaper. This is better than a screenshot as it takes the real image and not a scaled version of it. It can also fetch home and/or lock screen wallpapers, as well as other things the app offers.
LWP+ - a live wallpaper app that adds some extra features to just showing some content (image/animation/color) . Don't set my app as live wallpaper yet, and instead go right to its settings screen, and choose to show the current wallpaper in full screen. Then take a screenshot. Sadly this will work only for the home-screen wallpaper.
