Add a folder scan images there

def add_folder(self, folder):
        """ Add a folder scan images there """
        if folder in self.folders:
            return

        self.folders.add(folder)
        
        for subfolder, junk, filenames in os.walk(folder):
            for filename in filenames:
                name, ext = os.path.splitext(filename)
                if ext in self.exts:
                    self.images.append(
                        os.path.join(subfolder, filename))