//global variables that can be used by ALL the function son this page.
var inputs;
var imgFalse = 'unchecked.gif';
var imgTrue = 'checked.gif';

//this function runs when the page is loaded, put all your other onload stuff in here too.
function init() {
 replaceChecks();
}

function replaceChecks() {

 //get all the input fields on the page
 inputs = document.getElementsByTagName('input');

 //cycle trough the input fields
 for(var i=0; i < inputs.length; i++) {

 	//check if the input is a checkbox
 	if(inputs[i].getAttribute('type') == 'checkbox') {

 		//create a new image
 		var img = document.createElement('img');

 		//check if the checkbox is checked
 		if(inputs[i].checked) {
 			img.src = imgTrue;
 		} else {
 			img.src = imgFalse;
 		}

 		//set image ID and onclick action
 		img.id = 'checkImage'+i;
 		//set image
 		img.onclick = new Function('checkChange('+i+')');
 		//place image in front of the checkbox
 		inputs[i].parentNode.insertBefore(img, inputs[i]);

 		//hide the checkbox
 		inputs[i].style.display='none';
 	}
 }
}

//change the checkbox status and the replacement image
function checkChange(i) {

 if(inputs[i].checked) {
 	inputs[i].checked = '';
 	document.getElementById('checkImage'+i).src=imgFalse;
 } else {
 	inputs[i].checked = 'checked';
 	document.getElementById('checkImage'+i).src=imgTrue;
 }
}

window.onload = init;