// simple head flip
void setup() {
  PImage src = loadImage("portrait.jpg");
  size(src.width*2, src.height);
  background(255);
  image(src, 0, 0);
  PImage dest = createImage(400, 400, RGB);
  int len = src.pixels.length - 1;
  for (int i=len; i>=0; i--) {
    dest.pixels[len-i] = src.pixels[i];
  }
  image(dest, 400, 0);
}