def is_image(filename):
    """
    Check if filename has valid extension

    :param filename:
    :return: boolean indicating whether filename is a valid image filename
    """
    filename = filename.lower()
    return filename.endswith(".jpg") \
           or filename.endswith(".png") \
           or filename.endswith(".jpeg") \
           or filename.endswith(".gif") \
           or filename.endswith(".bmp")