A Processing sketch to hide messages in images. It assigns letters to numbers which are then used to change the red values in a .png copy of the original.

ImageEncoder

IntList numbers;

void setup () {
numbers = new IntList();
String abc = ". abcdefghijklmnopqrstuvwxyz";
String message = "there is a place where the sidewalk ends and before the street begins and there the grass grows soft and white and there the sun burns crimson bright and there the moon bird rests from his flight to cool in the peppermint wind";
int n = 0;
for (int i=0; i<message.length();i++) {
    String letter = str(message.charAt(i));
    numbers.append(abc.indexOf(letter));
  }  
  
  size(400, 317);
  PImage Original = loadImage("Starry Night.jpg");
  Original.loadPixels();
  PImage Encoded = createImage(width,height,ARGB);
  for (int x=0; x<width;x++) {
    for (int y=0;y<height;y++) {
      int loc = x + y * width;
      float r = red(Original.pixels[loc]);
      float g = green(Original.pixels[loc]);
      float b = blue(Original.pixels[loc]);
      if ( n<message.length()){
      print(r + ": ");
      r = r + numbers.get(n);
      print(r + " ");
      Encoded.pixels[loc] = color(r,g,b);
      n++; } else {
      Encoded.pixels[loc] = color(r,g,b);      
      }
    }
  }
  Encoded.updatePixels();
  image(Encoded,0,0);
  save("Encoded.png");
}

void draw(){
}