You'd have to modify the draw() function somewhat...

1. boolean doCapture; // Add this global variable.
2. void draw() {
3.     opencv.read();
4.     image( opencv.image(), 0, 0 );
5.    
6.     // detect anything resembling a FRONTALFACE
7.     Rectangle[] faces = opencv.detect();
8.     // Assume that no capture will happen this time.
9.     doCapture = false;
10.     // check each found face to see if it meets the conditions for triggering a capture.
11.     for( int i=0; i<faces.length; i++ ) {
12.         // Is the face large enough? (at least 25% the total area of the screen)
13.         if( ((faces[i].width * faces[i].height) > (.25 * width * height)) ){
14.             // Is the face fairly centered? (contains the point in the middle of the window)
15.             if( faces[i].x < width / 2.0 && faces[i].y < height / 2.0 && faces[i].width > width / 2.0 && faces[i].height  > height / 2.0 ){
16.                 doCapture = true;
17.             }
18.         }
19.     }
20.     if( doCapture ){
21.        save( "lastCapture.jpg" );
22.     } else {
23.    
24.     // draw detected face area(s)
25.     noFill();
26.     stroke(255,0,0);
27.     for( int i=0; i<faces.length; i++ ) {
28.         rect( faces[i].x, faces[i].y, faces[i].width, faces[i].height );
29.     }
30. }
31. }

Look into save() for more information on renaming the filename each time.
You might also want to check that a certain amount of time has past between captures. You can use millis() for that.

Thanks, that was really helpful!