Changing the size should make no difference - I use Eclipse and had much bigger windows than 400x400.

The message from Eclipse is called a stack trace and it lists the method calls that led upto the exception. If we look at yours -


we can see that the exception was caused by something in the draw() method in your file tutorial.java.

BINGO just spotted the error

Here is your draw method (I have taken the liberty to tidy it up a little) the mistakes are in lines 5, 6 and 7. The variables width and height refer to the size of the window canvas NOT the size of the image. So if the canvas is bigger than the image you run of pixels and that is why it worked when you reduced the size of the canvas. The solution is to use img.width and img.height in lines 5-7

1. public void draw() {
2. //computer i would like to talk to the pixels
3.     img.loadPixels();
4.     //run through the picture Identifyign the RED : Green : Blue colours
5.     for(int y = 0; y < height; y++) {
6.         for(int x = 0; x < width; x++) {
7.             int loc = x + y*width;
8. ///these functions identify the colour involved
9.             float r = red(img.pixels[loc]);
10.             float g = green(img.pixels[loc]);
11.             float b = blue(img.pixels[loc]);
12.             //image processing would go here
13.             //set the display pixel to the image pixel
14.             pixels[loc] = color(r,g,b);
15.         }
16.     }
17.     updatePixels();
18. }