def wms100format(format):
    """
    >>> wms100format('image/png')
    'PNG'
    >>> wms100format('image/GeoTIFF')
    """
    _mime_class, sub_type = format.split('/')
    sub_type = sub_type.upper()
    if sub_type in ['PNG', 'TIFF', 'GIF', 'JPEG']:
        return sub_type
    else:
        return None
