PImage img;
img = loadImage("test.png");
size(628, 350);
imageMode(CENTER);
noStroke();
background(255);
for( int i = 0; i < img.height; i++ ){
  for( int j = 0; j < img.width; j++ ){
    color c = img.get(j,i);
    float gray = (red(c)+green(c)+blue(c))/3;
    stroke(gray,gray,gray);
    point(j, i);  
  }
}