Your Pictures folder is not a real folder; it is called a library which is a collection of folders.
You can choose which folders are included in the library by clicking "3 locations" under the title "Pictures library". This will bring up a window with a list of folders included in the library. You can add and remove folders from this list. [EDIT: Removing a folder from this list does not actually delete the folder; it just removes it from the library.]
For more information about libraries, you can click on "Learn more about libraries" in the window with the list of folders or read
this article.
