PImage kitten;
int gridSize = 6;

void setup() {
  size(480, 480);
  smooth();
  kitten = loadImage("kitten.jpg");
  noStroke();
  background(0);
}

void draw() {
  for (int i = 0; i < kitten.width; i += gridSize) {
    for (int j = 0; j < kitten.height; j += gridSize) {
      float greenVal = green(kitten.get(i, j));
      fill(greenVal);
      rect(i, j, gridSize, gridSize);
    }
  }
  // set grid size to minimum 6, max 20 (depending on mouseX)
  gridSize = 6 + (14 * mouseX/width);
}