def page_number2image_name(number, string="image", padding_size=4):
    """
    Utility function to format a number with a padding of size n.

    :param number: the number to format (int)
    :param string: the prefix to prepend (str)
    :param padding_size: the desired lenght of the resulting string (int)
    :return: a string, whose length == padding_size and in which the number
            of zeroes == padding_size - len(number)

    Usage example:

    >>> page_number2image_name(24)
    'image-0024'

    >>> page_number2image_name(24,padding_size=5)
    'image-00024'

    """
    return "{}-{}".format(string, str(number).zfill(padding_size))