// This draws the mirror image of the input image when button is pressed.
PImage img; 

void setup() {
  //img = loadImage("");
  //make the size the size of your image, which you will have to look up in your computer
  size(300, 300);
}

void draw() {
  if (mousePressed) {
    mirror(img);
  }
  else {
    image(img, 0, 0);
  }
}

// This creates the mirror on the screen of the given image.
void mirror(PImage img) {
  int mirrorx;
 
  // for each x,y set the image to the mirror x but the same y
  for (int x = 0; x < img.width; x++) {
    mirrorx = img.width - x - 1;
    for (int y = 0; y < img.width; y++) {
      set(mirrorx, y, img.get(x, y));
    }
  }
}