PImage img1, img2;
PImage msk1;
PImage icn1;

void setup() {
  size(400,300);
  background(0);
  img1 = loadImage("img01.jpg");
  img2 = loadImage("img02.jpg");
  msk1 = createImage(img1.width,img1.height,ARGB);
  icn1 = loadImage("icn01.jpg");
}

void draw() {
  image(img1,0,0);
  img2.mask(msk1);
  img2.updatePixels();
  image(img2,0,0);
}

void mouseDragged() {
  int left = mouseX-icn1.width/2;
  int top = mouseY-icn1.height/2;
  left = constrain(left,0,img1.width-icn1.width);
  top = constrain(top,0,img1.height-icn1.height);
  msk1.blend(icn1,0,0,icn1.width,icn1.height,
  left,top,icn1.width,icn1.height,LIGHTEST);
  msk1.updatePixels();
}