PImage img;
int pixelSize = 3;

void setup() {
  // load image
  img = loadImage("lincoln.png");
  
  // set window size
  size(img.width, img.height);
}

void draw() {
  image(img, 0, 0);
  
  // iterate through whole picture
  for (int x = 0; x < img.width; x = x + (int)img.width*pixelSize/100) {
    for (int y = 0; y < img.height; y = y + (int)img.width*pixelSize/100) {
      color col = get(x,y);
      int grayScale = (int)((red(col) + green(col) + blue(col)) / 3.0);
      // iterate through pixel groups
      for (int sx = 0; sx < img.width*pixelSize/100; sx = sx + 1) {
        for (int sy = 0; sy < img.width*pixelSize/100; sy = sy + 1) {
          // set whole pixel group to color of first pixel
          set(x+sx, y+sy, col);
        }
      }
    }
  }
}

void keyPressed() {
  if (keyCode == DOWN) {
    if (pixelSize > 1) {
      pixelSize = pixelSize-1;
    } else {
      pixelSize = 1;
    }
  }
  if (keyCode == UP) {
    if (pixelSize <= img.width-1) {
      pixelSize = pixelSize+1;
    } else {
      pixelSize = img.width;
    }
  }
}