final int len=25;
final float thresh=170;

boolean newDesign=false;
PImage pic;

ArrayList<PImage> imgContainer;
int n=3;

void setup() {
  size(800, 800, P2D);
  colorMode(RGB, 255);
  background(250, 250, 250);
  rectMode(CENTER);
  //imageMode(CENTER);

  pic=loadImage("hand.jpg");
  pic.resize(width, height);
  pic.filter(THRESHOLD, 0.8);
  color c1 = color(200,25,25);
  color c2 = color(25, 255, 200);

  imgContainer=new ArrayList<PImage>();
PImage pimg1=loadImage("test1.png");
pimg1.resize(25, 25);
imgContainer.add(pimg1);
PImage pimg2=loadImage("test2.png");
pimg2.resize(25, 25);
imgContainer.add(pimg2);
PImage pimg3=loadImage("test3.png");
pimg1.resize(25, 25);
imgContainer.add(pimg3);
  noLoop();
  noStroke();
}

void draw() {
  if (newDesign==false) {
    return;
  }

  pic.loadPixels();

  for (int y = 0; y < height; y+=18) {
    for (int x = 0; x < width; x+=18) {
      // Get the color stored in the pixel
      int index=y*width+x;
      color pixelValue = pic.pixels[index];
      // Determine the brightness of the pixel
      float pixelBrightness = brightness(pixelValue);

      float imgPicked=constrain(pixelBrightness/thresh, 0, n-1);
      image(imgContainer.get((int)imgPicked),x,y);

    }
  }
}
void mouseReleased() {
  newDesign=!newDesign;
  redraw();
}