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

//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;