You might have noticed that the Magnets app is not asking you for permission to access your photo library when you select a new picture to upload.
The reason for that is that the app doesn't actually have access to your full photo library. Instead, when you tap on "Add Photo", iOS displays you an overview of all your photos. Magnets only gets access to the picture you select, and doesn't see any other of your pictures.
To learn more about how this works, check out Apples "link placeholder here"