Create an animation in Processing using PImage

I want to create a simple processing sketch ([login to view URL]). The function would be to draw an image on top of the previous image using the mouse clicked function. I have gotten half the code to work but I am unable to complete the clicking and placing the new image onto the existing one. I have attached the code file. I need to finish this project by monday march 23, 2015. Also as it is a class lab no plagiarism. Thanks