• In your code, you should put picture. in front of statements like loadPixels(), pixels.length and pixels[index]. Otherwise you are working with the pixel array of the main sketch window, while you wanted to work with the pixel array of the image.
• You could use picture.loadPixels, however for PImages it's not required. Those pixels are already loaded automatically.
• The pixel array stores ints. See an explanation of what those weird negative values mean in the wiki here.
• So you could print the int or get the red, green, blue, alpha values from the color.

You don't describe your end goal, but here is an adapted code example that shows how to do some of the described things. Hope it helps. Also check out the learning section on pixels.

1. PImage picture;
2.  
3. void setup(){
4.   String filename = selectInput();
5.   picture = loadImage(filename);
6.   size(picture.width, picture.height);
7.   int[][] pixel2DArray = new int[picture.height][picture.width];
8.  
9.   for (int i=0; i<picture.pixels.length; i++) {
10.     int y = i/picture.width;
11.     int x = i - y * picture.width;
12.     pixel2DArray[y][x] = picture.pixels[i];
13.   }
14.  
15.   println("INT: " + pixel2DArray[100][2]);
16.   println("red: " + red(pixel2DArray[100][2]));
17.   println("green: " + green(pixel2DArray[100][2]));
18.   println("blue: " + blue(pixel2DArray[100][2]));
19.   println("alpha: " + alpha(pixel2DArray[100][2]));
20. }
21.  
22. void draw(){
23.   image(picture, 0, 0);
24. }