writeBMP

import struct

def writeBMP(filename, matrix):
    height = len(matrix)
    if height < 1:
        raise Exception('Height must be >0')
    width = len(matrix[0])
    if width < 1:
        raise Exception('Width must be >0')
    data = sum(reversed(matrix), [])
    with open(filename, 'wb') as file:
        file.write(struct.pack('<H', 0x4D42)) # Signature.
        file.write(struct.pack('<I', width * height * 4 + 54)) # File size
        file.write(struct.pack('<III', 0, 0x36, 0x28)) # Offset and other tech data.
        file.write(struct.pack('<ii', width, height))
        file.write(struct.pack('<HH', 1, 0x20)) # Tech data and bitcount.
        file.write(struct.pack('<IIiiII', 0, 0, 0, 0, 0, 0,)) # Zeroes.
        for pixel in data:
            r, g, b = pixel
            file.write(struct.pack('<BBBB', b, g, r, 0xFF))