Clicking an Image to change it to another image

I'm very new to processing and have been trying to figure this out for awhile and I'm sure it is easy but I need help.

All I'm trying to do is turn the jellyfish image to the plastic bag image when it is clicked. I have a couple of jellyfish spread out through the sketch but wanted them to only changed when they are clicked indivdually. Here is the code that I have been working with, I'm very confused on what to put into the void mousePressed()

PImage ocean; PImage jellyfish; PImage bag;

void setup(){ size(1280, 520); //background image PImage ocean; ocean = loadImage("ocean.jpg"); background(ocean);

jellyfish = loadImage("jellyfish.png"); bag = loadImage("plastic_bag.png");

}

void draw(){ image(jellyfish, 500, 120); image(jellyfish, 600, 320); image(jellyfish, 100, 20); image(jellyfish, 1000, 50); }

void mousePressed(){

}

}