Image Swap & Count Apparently - Assign "add 1 When Clicked" Or "subtract 1 When Clicked" To Image Properties?

I have a very simple web page that is comprised of the same image over and over again. I have it to when you click on it, it swaps the image out and adds 1 to the counter. I need it to subtract 1 when you click it again.

<script type="text/javascript">
var namesVec = new Array("block.png", "block_hover.png");
var root='images/';
function swapImg(ima){
// divides the path
nr = ima.getAttribute('src').split('/');
// gets the last part of path, ie name
[Code]...

I mean, is there a way to assign "add 1 when clicked" or "subtract 1 when clicked" to image properties?