List files from dir_path and register images with
            filename as key (without extension)
        Additionaly a prefix for the key can be provided,
        so the resulting key will be prefix + filename

def register_from_dir(cls, dir_path, prefix=''):
        """List files from dir_path and register images with
            filename as key (without extension)
        Additionaly a prefix for the key can be provided,
        so the resulting key will be prefix + filename
        """

        for filename in os.listdir(dir_path):
            name, file_ext = os.path.splitext(filename)
            if file_ext in cls._formats:
                fkey = '{0}{1}'.format(prefix, name)
                cls.register(fkey, os.path.join(dir_path, filename))