def split_image_name ( image_name ):
    """ Splits a specified **image_name** into its constituent volume and file
        names and returns a tuple of the form: ( volume_name, file_name ).
    """
    col         = image_name.find( ':' )
    volume_name = image_name[ 1: col ]
    file_name   = image_name[ col + 1: ]
    if file_name.find( '.' ) < 0:
        file_name += '.png'

    return ( volume_name, file_name )