I know this is not directly answering your question, but you could store a copy of pixels[] / the image in its original state. When you want to modify the image use the original in the for loops but store the changes in pixels[] so that the copy never changes. Like this:

I tried doing that with arrayCopy, but processing didn't like it... That wouldn't be exactly what I'm looking for, but it would be a start, since I could progressively change the values back to their original state. 

You could just do it manually once in setup() with a for loop. Create an array that is the size of width*height, display your image in setup() to give pixels[] the appropriate values, use the for loop to set every element of the array to be the same as every element of pixels[], and then continue with your program. Then you would never change that array but use it as reference for the original state of your image.

True... still not the ideal solution for me though.

To change the brightness you can also use a black/white rect. The advantage of this method is that it's very low on resources and allows quick non-destructive changes between dark/light. The disadvantage of this method is that you don't actually change the brightness of the image itself (only the appearance).

Code Example

1. PImage img = loadImage("http://mw2.google.com/mw-panoramio/photos/medium/11429567.jpg");
2.  
3. void setup() {
4.   size(img.width, img.height);
5. }
6.  
7. void draw() {
8.   image(img, 0, 0);
9.   float w1 = mouseX/(width/510.0);
10.   float w2 = width/(width/255.0);
11.   fill(mouseX<width/2?0:255, mouseX<width/2?w2-w1:w1-w2);
12.   rect(0, 0, width, height);
13. }