//global variables that can be used by ALL the functions on this page.
var inputs;
var imgFalse = 'img/checkbox_off.gif';
var imgTrue = 'img/checkbox_on.gif';

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

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;