Do you have a pressed() method defined outside of the Button class [same scope as draw() or setup()]?

Could you post some runnable code for us to look at?
It would make it easier to track down.

Thanks Smitty,
That did it! I was trying overly complicated examples that I could have tried out had I more time but this does what I want it to do and with little complication.
This is a small part of a larger project I'm working on and if anyone's interested I'll be giving away my final code (I'm doing a final year BA at uni) for anyone that wants it.
Thanks very much,

Tim

Okay, let me make sure I understand:

You're trying to create a set of buttons corresponding to strings in the array cellNames[].  You want to place these buttons in a square array, but there's no guarantee that the number of strings in cellNames[] is a perfect square.  You're running into errors trying to populate a square grid with a non-square number of strings in cellNames[].

Is that correct?

Assuming I'm not misunderstanding you, I suggest the following changes to your code:
Code:

    grid=new ImageButtons[cols][rows];
    for(int i=0; i<cols; i++){
	for(int j=0; j<rows; j++){
	  int c = i*rows + j;  // Note 1
	  if (c < cellNames.length){  // Note 2
	    PImage tmp;
	    tmp=loadImage(cellNames[c]);
	    println(cellNames[c]);
	    grid[i][j]=new ImageButtons(i*cellWidth, j*cellHeight, cellWidth, cellHeight, cellNames[c], tmp);
	    println("success");
	  }
	}
    }
 

Note 1: Doing this will use each element of cellNames[] exactly once, which I think is what you want.

Note 2: This line checks to see if cellNames[] is big enough to contain an element for this grid box (which I think is what you were trying to do when you were checking for a null value before, but that way doesn't work.)