Where to find a picture from photo library on the file system?
Oddly enough you did get the physical location of the picture.

It is inside the Photos Library.photolibrary package

While you can open that package file by right-clicking or control-clicking on that file and selecting "Show Package Contents" it is generally not a good idea to do this as just deleting the original will not remove it from the Photos.app. In other words the next time you launch Photos that picture will seemingly still be there.

Your best bet is to delete the picture from within the Photos.app and stay out of the photolibrary package.