Why is System Preferences not recognising Pictures folder? [macOS 10.13.4] I want to change my 2016 MacBook Pro's wallpaper to a custom image which is already in the Pictures folder.

If I launch System Preferences &gt; Desktop &amp; Screen Saver &gt; Desktop, under Folders there is the Pictures one but it looks empty. 
Is it normal?

Only workaround by now is to click the '+' button and add the picture manually which, strangely, adds all of the pictures contained into the Pictures folder. 
NB: into my Pictures folder there are two custom created  subfolders which gets ignored by this last operation.

What is going wrong here?
Yes, this is normal.

This folder corresponds to the immediate (not including sub folders  folder located at ~/Pictures. 

You can access this folder by clicking Pictures in the Finder sidebar or by going to Finder>Go>Go to Folder> ~/Pictures>Go

See pictures below

Example 1




Example 2