Windows 10 - Change pre logon wallpaper to windows spotlight

Unfortunately Microsoft haven't made that entirely possible , however you could try a method which I thought of.

Method:

Well since the location of both images (Login and Lock screen) are visible, you could get the images from Lock screen to replace one on login.

1. Navigate to the Lock screen backgrounds with this directory:

   C:Users\{your user name}\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState\Assets

   You may have to enable "Show Hidden Files" in Control Panel>File Explorer Options

2. Copy all the images and make a folder on the desktop. Paste them into there. Rename the images to whatever.jpg

3. Read and follow this article until you get to Setting your image however take a note if the backgrounds directory

4. Take ownership of the backgrounds directory for login (search it up in Google).

5. You can then replace them with the backgrounds from the folder in the desktop

To get them change automatically you can work on a cmd script to randomise each time you log in.

Edit: Also here is a link of where you can get the location of the Lock Screen backgrounds:

http://www.tenforums.com/tutorials/5016-lock-screen-background-change-windows-10-a.html