Try this:

1. PImage getFrame() {
2.   PImage img = new PImage(width, height);
3.  
4.   g.loadPixels();
5.   img.loadPixels();
6.  
7.   img.pixels = g.pixels;
8.  
9.   img.updatePixels();
10.   g.updatePixels();
11.  
12.   return img;
13. }

Or...

1. PImage screen = get();

I haven't tried, but I think calsign's method isn't a snapshot, there is no copy of pixels, it will be updated on the next frame.
amnon.owed's method is better for a snapshot.