A Processing sketch to pull messages out of pngs encoded with the ImageEncoder at https://gist.github.com/gifguide2code/382e99ee16e9defdaa9185979c3242ae

ImageDecoder

IntList numbers;
void setup () {
  numbers = new IntList();
  size(400, 317);
  PImage Original = loadImage("Starry Night.jpg");
  PImage Encoded = loadImage("Encoded.png");
  Original.loadPixels();
  Encoded.loadPixels();
  for (int x=0; x<width;x++) {
    for (int y=0;y<height;y++) {
      int loc = x + y * width;
      float rOrig = red(Original.pixels[loc]);
      float gOrig = green(Original.pixels[loc]);
      float bOrig = blue(Original.pixels[loc]);
      float rEnc = red(Encoded.pixels[loc]);
      float gEnc = green(Encoded.pixels[loc]);
      float bEnc = blue(Encoded.pixels[loc]);
      if ( (rOrig != rEnc) ){
      numbers.append(int(rEnc - rOrig));
      } else if (gOrig != gEnc) {
      numbers.append(int(gEnc - gOrig));        
      } else if (bOrig != bEnc) {
      numbers.append(int(bEnc - bOrig));      
    }
  }
  }
  String abc = ". abcdefghijklmnopqrstuvwxyz";
  for (int i=0; i<numbers.size();i++) {
    int num = numbers.get(i);
    print(abc.charAt(num));
  }
}
void draw(){
}