Answer:
How to find out what the Windows 10 login background picture is:
- Go to where the picture is stored and save a copy by...
- C:\Users\<username>\AppData\Local\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState\Assets
- copy files and rename to end in .jpg
- Reverse Image Search by...
- Open Google
- Select "Images" (upper right corner area)
- Select the camera icon in the search widget window (Note: the default tab already selected is "Paste image URL")
- drag the image you want to search for from the location you copied it to, and drop the image into the Google Image search window
- Google will find the image, tell you what it is, etc.