Is it possible to avoid 2 loops inside another loop?

[Gribouillis]

You could simplify things with os.walk()

import os

def _rename_images(self):
    for folder in self._valid_folders():
        self._rename_and_move_images(...)

def _valid_folders(self):
    for folder in self._subfolders(self._root_source):
        dirnames, filenames = next(os.walk(folder))[1:]
        if filenames and not dirnames: # folder not empty and without subfolder
            yield folder

def _subfolders(self, folder):
    for dirname in next(os.walk(folder))[1]:
        yield folder / dirname