import re


def find_image_filename(input_image_dict, filename):
    """
    Helper function to retrieve the full filename of the specified image using the provided dictionary.

    :param input_image_dict: the dictionary containing the full filename of available images.
    :param short_filename: the unique short filename of the image to use to look up the full filename in the dictionary.
    :return: the full image filename.
    """
    short_filename = filename if "/" not in filename else re.split("([^/]+$)", filename)[1]
    return input_image_dict[short_filename]