Microsoft decides what the default folder on a Windows device will be for pictures. I know a lot of camera manufacturers use folder names like DCIM and I am happy at Microsoft they choose to name it "Pictures". And that makes sense.
I don't need to install a OneDrive app on my Windows Mobile device, because that is all integrated.
However, with the last part of your post we're getting somewhere! Yes, I have set the Camera app to sync with OneDrive. I have done so on the phone of my mother in law as well. However, on my setup the pictures ended up in the root of OneDrive in the folder "Files" --> "SkyDrive camera roll". On the setup of my mother in law it was worse and the pictures ended up in: "Files" --> "familie foto's" --> "Harma" --> "Pictures" --> "Verkoop" --> "familie foto's". That's why I was looking for ways how to change that.
Now, as mentioned in my second post, I arranged for her pictures to end up in " Files" --> "Pictures" --> "Camera uploads"