example

PImage img;
int stepSize=10;

void setup() {
  size(100, 100);
  img = loadImage("image1.jpg");
  surface.setResizable(true);
  surface.setSize(img.width, img.height);
  for (int x=0; x<width; x+=stepSize) {
    for (int y=0; y<height; y+=stepSize) {
      color c = img.get(x, y);
      fill(c);
      noStroke();
      ellipse(x, y, stepSize, stepSize);
    }
  }
}