Following up because it took me a while to figure out.
https://github.com/kolbyjack/MMM-Wallpaper
You use the iCloud album id as a config item.
source: “icloud:”,
The album id is simply the last bit in your iCloud shared album URL. It will look something like this:
https://www.icloud.com/sharedalbum/#nUm83r5N13t+3r5
That’s not a real link, just an example.
Copy everything to the right of the #, basically nUm83r5N13t+3r5.
You must use a shared album and you have to set it to Public Website (anyone with the link can view the photos). That will allow you to generate the link. See: https://support.apple.com/en-us/108314
This is what a basic config will look like.
modules: [ { module: "MMM-Wallpaper", position: "fullscreen_below", config: { // See "Configuration options" for more information. source: "icloud:nUm83r5N13t+3r5", slideInterval: 60 * 1000 // Change slides every minute } } ]