In order to use the Apple iCloud Album Integration, you need publicly shared Apple iCloud Photo Album. Set source to iCloud and put the URL or ID of the photo stream into the path setting. Example URL: https://www.icloud.com/photostream/#A2GWZuqDGlcU43

For more information how to create a public photo stream, have a look here: https://support.apple.com/kb/PH12068

Displaying hundreds of photos could be quite time consuming. Therefore, it is possible to limit the amount of photos to be loaded from the photo stream.