import pyAesCrypt
from os import stat, remove

bufferSize = 56 * 1024
password = "itisjustastring"

encFileSize = stat("justanimage.png.aes").st_size

with open("justanimage.png.aes", "rb") as fIn:
    with open("dataout.png", "wb") as fOut:
        pyAesCrypt.decryptStream(fIn, fOut, password, bufferSize, encFileSize)

bufferSize = 56 * 1024 - from picture
password = "itisjustastring" - from picture