I use a screensaver on my Mac, and it seems like it refuses to recognise the image in ~/Library/Application Support/Artpip/images as an inmate without the .jpg extension. Is there any chance of you changing the images to use a .jpg extension, so I can set my screensaver to use your folder automatically.

???

http://www.kunal-chowdhury.com/2015/11/uwp-lock-screen.html

from what I recall you are able to set it from the registry.

create this key with a string value called 'LockScreenImage', this string should point directly to an image, i.e: C:\.... HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Personalization

throwaway because of not wanting to get arrested for hacking the registry