Probably should be:

1. void mousePressed() {
2.    imageIndex = int(random(images.length-1));
3. }

The index starts at 0, but the length starts at 1.  So a length of 3, will have an index of 0, 1, 2.  What it looks like you did here was allow an index = length, which would produce a null since the max index is length-1.

I suspect it's the images' filenames. Do you name them as "Ball 10.jpg" and "Ball 11.jpg"? Otherwise, it will give you an error.

You could wrap it in an if statement for troubleshooting.

1. if (images[imageIndex] != null) {
2.    image (images[imageIndex], 0, 0);
3. } else {
4.    println(imageIndex + " is reporting null.");
5. }

As said, verify the files in the data folder have exactly the same name, including case and spaces.