Pointless

Gimp? No. You can do this in flash.
Import the spritesheet, edit it, break it apart and use the magic wand tool to delete the background. Due to that, this whole tutorial it utterly useless, not to mention badly made (hiding the cursor for no reason? -.-).

Incredible

I'm afraid I'm going to have to go for the corny 20 out of 10 starts here :P
That was an amazing piece of.. Art? Animation? Philosophical ideas? I'm not really sure, but it was great never-the-less. The art style with smooth lines and emphasized faces really went with the seemingly metaphorical story.
It's great that you got hold of a music track built around the flash, it made it far better than if you had built the flash around the music. If I could, I would tell you how to improve. But I can't :P Incredible flash, good work.

Wow.

Great =P
Unique graphics, with great voice acting.
Funny story, and orginal concepts.
And the credits song..
Heh, ...That doesn't make any sense... ...Hi Mum!...
That is destined to get in the top 50.
Make many, many more.

Hmm.

I personally like this game, but I have a few gripes..
Changing sectors resets your weapon to the lowest number. That's just really annoying. Also, couldn't you have weapon up/down buttons just above fire? It's annoying and slow to move your whole hand to cycle through your weapons. Also, when you die, you need an immortality couple of seconds. I once lost 3 lives in quick succession by spawning next to the same battleship repeatedly.
Some of the weapon info's are not working, and it would be better if there was more variety in missions. Eg. smuggling contraband into a station while trying to avoid law enforcers who will detect you and become hostile if you get too close. All in all it was fun, but it could be far better.

Amazing

I have been waiting for a game like this for ages. Utterly simple but also very complex. The way you played nothing but the music throughout, ignoring things like the gunshot, was very effective. The music worked well with the graphics. The hand-drawn graphics were great, I've seen nothing like them before. They brought life to the game. The puzzles were nice, and some of them were quite funny, like the parachute pack. I didn't get the numbers one for a minute or two, and then saw the answer numbers while looking for another route. The setout of the scenes was great, with ink spodges to fill space.

...SEQUEL TIME~!

You made me write a rant!

Amazing.
To cut to the chase:
Bug:
I found a bug. After going through one game and dieing, when starting the next one as soon as someone let me at a uzi it was fully upgraded with rapid fire and infinate ammo. Seems like it forgot to reset. You might wanna fix that.
Suggestions:
Yes, in this geometric style game a level editor would make this
incredible game a legacy. Create the floor, zombie spawn points, and add items. You should be able to save and download others' levels too.
I want a new weapon: Flamethrower. One of the last weapons.
Short range but wide angle, eats up ammo quickly.
Has an upgrade, "wider flamethrower"
And: Sword. A mid level weapon, the bounus being that it has infinate ammo all the time. Short range but fast, one chop kill. Not very good against a croud, but useful for small groups.
Every now and then, I would like to be able to choose an upgrade for my Box Head =P. Just something like an extra life, a bit faster movement, or perhaps a score modifier. It could pop up in a corner of the screen with hotkeys under each one. Say, one upgrade every 50 zombies to start with, but it gets higher.
An unlockable secret mode, with all weapons, an uber weapon, much faster spawn rate and more violence. You must get a good score on all of the normal levels to play it.
When you kill a certain number of zombies quickly, activate the ranks, along with a 2nd timer for abotu 30 seconds. You must kill enoguh zombies in the next 30 seconds to advance to the next rank. Perhaps ranks have boosts to them as well.
Oh, and please.
Allow zombies to eventually push a barrel away if they're not killed for long enough. Barreling off sections is like a cheat
x.x

But I'm done. Please consider these suggestions in a sequel (which you will make or I will uzi you) and keep up the good work!