My image editing app is saving some important data in the documents directory. In a tutorial I was reading this:
iTunes will then display anything you
save to the Documents directory in
your app to the user, when they go to
the “Apps” page in iTunes and scroll
to the bottom:

I have a subfolder called userImages and it would be clever to restrict file sharing only to that folder and not to everything in documents. Otherwise the user would accidently (or on purpose) mess around with files that the app depends on to work properly. This would be bad.
Is there a way to restrict it to a subdirectory in documents?
No, what you should do instead is store anything you do not want users seeing in the “Library” directory for the app. Check here for a list of places you can store data:
How can I get a writable path on the iPhone?