PImage a1,a2;
int a,b,c; 



void setup() 
{
  size(700,500);
  frameRate(100);
  a1 = loadImage("Tag.jpg");
  a2 = loadImage("Nacht.jpg");
  image(a1,0,0);
}

void draw() 

{
  tint (255,255,255,100); 
  if (mousePressed == true)
  {
  PImage b = a2.get(mouseX, mouseY, 30, 30); 
  image(b, mouseX, mouseY); 
  }  else {
         PImage c = a1.get(mouseX, mouseY, 30, 30);
         image(c, mouseX, mouseY);
                
        }

}