http://processing.org/reference/PImage_set_.html

Syntax
img.set(x, y, c)
img.set(x, y, img)
Parameters
img PImage: any variable of type PImage
x int: x-coordinate of the pixel
y int: y-coordinate of the pixel
c int: any value of the color datatype
img PImage: image to draw on screen

1. you're using the same variable name "img" for two variables
2. "img: image to draw ON SCREEN"?!?!?!?!? doesn't make any sense. Looks like something that got copy-paste from some other function's doc.