PImage b;

void setup() {
  size(640,480);
  noStroke();
  b = loadImage("Photo13.jpg");
}

void draw() {
  background(b);
  int mx = constrain(mouseX,0,639);
  int my = constrain(mouseY,0,479);
  loadPixels();
  color c = pixels[my*width + mx];
  fill(c);
  ellipse(320,240,90,90);
}