PImage img;
String url = "http://processing.org/img/learning/reas-book.jpg";

void setup() {
  img = loadImage(url, "jpg");
  size(img.width, img.height);
  int wh = img.width * img.height;
  for (int i=0; i<wh; i++) {
    img.pixels[i] = 0xffffff - img.pixels[i];
  }
  image(img, 0, 0);
}