Transparency.

Move the pointer left and right across the image to change its position. This program overlays one image over another by modifying the alpha value of the image with the tint() function.

PImage a, b;
float offset;

void setup() {
  size(200, 200);
  a = loadImage("construct.jpg");  // Load an image into the program 
  b = loadImage("wash.jpg");   // Load an image into the program 
  frameRate(60);
}

void draw() { 
  image(a, 0, 0);
  float offsetTarget = map(mouseX, 0, width, -b.width/2 - width/2, 0);
  offset += (offsetTarget-offset)*0.05; 
  tint(255, 153);
  image(b, offset, 20);
}