In theory each pixel in the image could be meeting your condition. However your array is only the size of a single line of pixels. So clearly the array is way to small, which is why it quickly goes out-of-bounds. Your workaround isn't working very well either. So when you want to store stuff, but you don't know exactly the number beforehand, it's easier to use an ArrayList. You can add things to that without knowing the exact size of the arrayList beforehand. Also in this case I would use an ArrayList of PVectors so you can easily store sets of xy coordinates.

In a more general sense, note that your threshold condition is relatively low and usually there are many pixels next to each other of a higher brightness. So in most images, your code will result in an overkill of overlapping ellipses. Because each 1x1 bright pixel is turned into an 10x10 ellipse. Perhaps you have a specially prepared image where this is all right.

Anyway, here is the adapted code which uses an arraylist of PVectors. The image is analysed in setup() once. In draw() the image and the ellipses are displayed. Press the mouse to see the original image without ellipses.

Adapted Code

1. PImage img;
2. ArrayList <PVector> points = new ArrayList <PVector> ();
3.  
4. void setup() {
5.   img = loadImage("http://poster-posters.nl/images/gb_lp1443.jpg");
6.   size(img.width, img.height);
7.   for (int y=0; y<img.height; y++) {
8.     for (int x=0; x<img.width; x++) {
9.       int index = x + y * img.width;
10.       color col = img.pixels[index];
11.       if (brightness(col)>100) {
12.         points.add(new PVector(x, y));
13.       }
14.     }
15.   }
16.   smooth();
17. }
18.  
19. void draw() {
20.   image(img, 0, 0);
21.   if (!mousePressed) {
22.     noStroke();
23.     fill(255, 0, 180, 13);
24.     for (PVector p : points) {
25.       ellipse (p.x, p.y, 10, 10);
26.     }
27.   }
28. }

Thank you for the quick response.

Like this:

1. beginShape();
     for (int i =0 ; i< points.size(); i++) {
       PVector p = points.get(i);
       vertex(p.x, p.y);
     }
     endShape();

ok cool. we'll see what happens. thank you for your help!