You can use get() on an image to get a section of it. You can use the width and height of the sketch as well as the width and height of the images to place things correctly. See the example below.

Code Example

1. PImage img = loadImage("http://www.britishmolecatchers.co.uk/images/square007.jpg");
2.  
3. void setup() {
4.   size(500, 500);
5. }
6.  
7. void draw() {
8.   background(255);
9.   int h = constrain(height-100-mouseY, 0, img.height);
10.   PImage section = img.get(0,0, img.width,h);
11.   int x = (width-img.width)/2;
12.   int y = height-100-section.height;
13.   image(section, x, y);
14. }