How to transfer images from macOS to the primary Photo Library on iOS? Is there any method for transferring photos from a folder directly into the Photo Library (main library, not an album) on iOS from macOS? 

I have a situation where I need to use an app (Scanner Pro) on iOS to manipulate scanned images from macOS, however each time I push them to my iOS device via iTunes by selecting a folder to sync to iOS from my Mac, they are not showing up in the app supposedly because they're in an album on iOS and not in the main "Photo Library."
I always use AirDrop to transfer things between my macOS and iOS devices, and it works very well.