shouldnt be to hard.
You load two images, use a loop to check every pixel using get or the pixels array of the images and if the colors are different draw a point to your screen.

So writing pseudo code it could look something like this.

size( imageWidth,imageHeight);

for(int x = 0; x<width; y++){
for(int y = 0; y<height; y++){
if(image1.get(x,y)!=image2.get(x,y)) point(x,y);
}}

doing something like this you create an image of the regions that are different.

super new to the processing world and I am working on a project to compare video with a database of pictures, and if the pictures are similar I would like to output "success" or something similar.