Blending images, and keepin white as a "blank" value

Might be hard for me to explain exactly what I mean, but here goes.

I'm kinda new to all this. I get max patches to work, by copying pieces of tutorials together and making smaller add ons. Point being, I'm not fluent in the max language. Even worse in jitter.

What I'm trying to do, is to mix/crossfade/blend several images. The goal is as many as 16. The images are drawing, i.e. white background with pencil like drawing on them. I've managed to make a scaling sub patch, that makes sure, that the sum of all the image faders doesn't exceed 1. But the white background of one image blurs or fades the drawing of another image.

What I'm wondering: Is there a way to tell the jitter matrix, that white is only background. So that when two images are blended the black information of one replaces the white information of another.

These images are real pencil drawings that I scan into a jpeg and make a small collage of similar motifs wich them are made into an .avi to loop. Then I have several small movies that are faded in and out on top of each other based on different parameters.

And thx so far, for all the help I've found on this forum already.