REDUCING LAG FROM BIG PICTURES?

Hello,

I was wondering if there's a way to reduce lag from "big" (960x960) pictures? I am using Modern Algebra's "Fixed Pictures" script for use with pictures for some sort of "overlay" mapping. I'm basically only adding shadows and lighting effects but one particular dungeon has lots of smaller lights, lots of colours and tons of little shadows and that dungeon lags more than any other.
I thought a 30x30 map would be small enough but apparently even that is not something the engine can handle.

I'm also wondering how everyone who parallax maps a lot copes with this since I assume it's even more resource intensive since you have like 2-3 layers? Or perhaps there's a different script that uses the pictures differently so it lags less?