Delete unused images

def _cleanPictures(self):
        """
        Delete unused images
        """

        # Project have been deleted
        if not os.path.exists(self.path):
            return
        try:
            pictures = set(os.listdir(self.pictures_directory))
            for drawing in self._drawings.values():
                try:
                    pictures.remove(drawing.ressource_filename)
                except KeyError:
                    pass

            for pict in pictures:
                os.remove(os.path.join(self.pictures_directory, pict))
        except OSError as e:
            log.warning(str(e))