Einstein exploding

PImage img;
ArrayList pieces;
float x, y;
float incr;
color col;
int counter = 0;

void setup() {
  img = loadImage("http://i.imgur.com/GypezS6.jpg");
  size(img.width, img.height);
  img.loadPixels();
  rectMode(CENTER);

  pieces = new ArrayList();
  incr = 2;

  for (int i = 0;  i<width; i += incr) {
    for (int j = 0; j < height; j += incr) {
      x = i + incr/2;
      y = j + incr/2;
      col = img.pixels[int(y)*width+int(x)];
      Piece piece = new Piece(x, y, col);
      pieces.add(piece);
    }
  }
}

void draw() {
  background(0);
  for (int i = 0; i < pieces.size(); i++) {
    Piece p = (Piece) pieces.get(i);
    p.run();
  }
}

class Piece {
  float x, y;
  float incr_ex = 0.01;
  color c;
  PVector s;

  Piece(float _x, float _y, color _col) {
    x = _x;
    y = _y;
    c = _col;
    s = new PVector(random(-2, 2), random(-2, 2));
  }

  void run() {
    display();
    if (frameCount > 75) {
      explode();
    }
  }

  void explode() {
    x += s.x;
    y += s.y;
  }

  void display() {
    fill(c);
    noStroke();
    rect(x, y, incr, incr);
  }
}