#!/usr/bin/env python3

with open('challengefile', 'rb') as file:
    content = file.read()

def chunks(l, n):
    for i in range(0, len(l), n):
        yield l[i:i + n]

buffer = []
for word in chunks(content, 4):
    buffer.append(bytes(reversed(word)))

with open('challengefile.jpg', 'wb') as file:
    file.write(b''.join(buffer))