This is not much steganography and by ditching steganography completely you could make it so that the file is both a valid image and a valid zip.
At least that would make it easy to extract the data.
Needless to say that too has already been done many times in the last 20 year.
Stenography is quick writing using a vowel-/tone-based script usually used to record the spoken word. You are thinking about steganography, which is hiding something inside other data.
The ZIP thing is a nice trick, but by now is being checked for regularly when computer forensics come into play. Also, I would argue that it is not actually steganography, for it does not hide itself.
That's just about "hiding" being a relative word. "ZIP thing" also can be considered hiding, if you are absolutely clueless. And if we speak about "forensics" — that toy implementation is about as much "hiding" as the ZIP thingy. For more or less solid tool somebody already mentioned steghide here, as well as the reasons why the OP's script doesn't really hide anything, if we are being meticulous.
Needless to say that too has already been done many times in the last 20 year.