That's pretty smart :)

Using the image you mentioned in your first message create a second black and white image as a mask (see below) the black areas indicate where you can have particles. The code below will draw 1000 circles on the original image the result is shown here.
HTH

1. PImage picture, pic_mask;
2. int[] pxls;
3. int PSIZE = 4;
4. 
   
5. void setup()
6. {
7.   size(540,540);
8.   picture = loadImage("fig8one.jpg");
9.   pic_mask = loadImage("fig8one.png");
10.   image(picture, 0, 0);
11.   drawParticles(1000, pic_mask);
12. }
13. 
    
14. void drawParticles(int nbrParts, PImage m)
15. {
16.   int x, y, p;
17.   m.loadPixels();
18.   ellipseMode(CENTER);
19.   fill(255,0,0);
20.   stroke(128,0,0);
    
21.   do {
22.     x = (int) random(m.width);
23.     y = (int) random(m.height);
24.     if(red(m.get(x,y)) < 128) { // will be 255 for white and 0 for a black pixel
    
25.       ellipse(x,y,PSIZE,PSIZE);
26.       nbrParts--;
27.     }
28.   }
29.   while(nbrParts > 0);
30. }