PImage img = loadImage("bieber.jpg");
size(img.width, img.height);
noStroke();

for (int y = 0; y < height; y = y + 1) {
  for (int x = 0; x < width; x = x + 1) {
     color c = img.get(x, y);
     float r = red(c);
     float g = green(c);
     float b = blue(c);
     float avg = (r + g + b) / 3;
     fill(avg, avg, avg);
     ellipse(x, y, 1, 1);
  } 
}