Re : Adding background image to existing code

In your code above line 51 is in the wrong place it should be in the draw() method so move it so it looks like this

1. void draw() {
2.   background(b);
3.   updateSerial();
4.   drawBoard();
5. }

Also you should remove the following line (93) from the drawBoard() method
  background(255);

since it will set the background to white, replacing your image.