This utility was created with the purpose of enabling the use of Windows Spotlight (lockscreen) images as desktop backgrounds. This function does not exist in Windows 10. However, the files are on your local disk, so it's just a matter of copying these to your wallpaper folder. This utility automates that process.
If you don't care about the source code, but just want the tool, browse to the bin\release folder and download JpegSync.exe. That's all you'll need!
JpegSync -i path Shows info on all JPEG files at the specified location -s path_from path_to Syncs JPEGs from first path to the second
Example
JpegSync.exe -s "%localappdata%\Packages\Microsoft.Windows.ContentDeliveryManager_cw5n1h2txyewy\LocalState\Assets" "D:\User\Pictures\wallpapers"
The application works the way that it reads all JPEGs at the path_from location and their dimension and file size. These attributes are compared to the files at the path_to location. If there is no match on these three attributes, the file is copied, hence adding the new lockscreen image to the wallpapers collection.
Currently, only landscape images in HD (1920x1080) are copied. This filter can easily be changed in the code in the Program.SyncPaths method.
To run this automatically at startup, add a shortcut to the application in your startup folder
C:\Users\[username]\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup