I started with one of the examples that comes with Processing added the G4P library (Select Sketch | Import Library | Add Library | G4P from the menu). I added a G4P button to the sketch, an event handler for the button and I end up with the code below.

BTW it works and saves a numbered image every time the button is clicked.

1. /**
2.  * Getting Started with Capture.
3.  *
4.  * Reading and displaying an image from an attached Capture device.
5.  */
6. import processing.video.*;
7. import g4p_controls.*;
8. 
   
9. Capture cam;
10. GButton btnSave;
11. int n = 1;
12. 
    
13. void setup() {
14.   size(640, 480, P2D);
15.   btnSave = new GButton(this, width-44, height-30, 38, 24);
16.   btnSave.setText("SAVE");
17.   String[] cameras = Capture.list();
18.   if (cameras.length == 0) {
19.     println("There are no cameras available for capture.");
20.     exit();
21.   }
22.   else {
23.     println("Available cameras:");
24.     for (int i = 0; i < cameras.length; i++) {
25.       println(cameras[i]);
26.     }
27.     // The camera can be initialized directly using an element
28.     // from the array returned by list():
29.     cam = new Capture(this, cameras[0]);
30.     cam.start();
31.   }
32. }
33. 
    
34. void draw() {
35.   if (cam.available() == true) {
36.     cam.read();
37.   }
38.   image(cam, 0, 0);
39.   // The following does the same, and is faster when just drawing the image
40.   // without any additional resizing, transformations, or tint.
41.   //set(0, 0, cam);
42. }
43. 
    
44. public void handleButtonEvents(GButton button, GEvent event) {
45.   if (button == btnSave) {
46.     saveFrame("frame_" + n + ".jpg");
47.     n++;
48.   }
49. }