Very good for flash.
You did a great job on making this and it's great for flashes. I would know since we used it in my Resident Hall Evil flash. I really don't think there is anyway to improve something of this sort. It's done very well. Great colors used and very good all around. The night sky that you added was a great addition and looks really cool along with the zombie.
The zombie is kind of cartoon, but if that was what you were going for then great. I think a more realistic zombie would have dried old skin instead of the green skin that you used, but I know where you were coming from for sure.
I think the real question now is could you animate that zombie fluently? Or would that be something out of your league? I would love to make a zombie flash someday with you if you could get something like this animated.