docs: 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))