Files for unrestricted use by the user, and certainly if to be used with other apps, go in documentsDirectory or userDirectory, but there's currently no distinction between them so pick whichever feels best to you.

If you don't mind some PB-specific support, consider adding one of the listed subfolders below the userDirectory, with things like File.userDirectory.resolvePath('photos/myimage.jpg').

App data that is to only be used with your app should be saved in the applicationStorageDirectory