Some of you more observant fellows will notice that on the Zombie Cave mission, a nifty blending effect is used on some of the layers to make them look sexier. I was wondering if there's some sort of blending or blurring tool that works on an index and on such a small scale, or if I'm doomed to editing them all by hand.

Oh yea, I'm using gimp

It's definitely something Photoshop-y but you can get the same result with the spay and lasso tools in Paint.
Good luck with that.