var checkedImage = new Image();
checkedImage.src = 'checked.gif';
var uncheckedImage = new Image();
uncheckedImage.src = 'unchecked.gif';

function toggle(imgObject, radioObject){
if(imgObject.src.indexOf('unchecked.gif')!=-1){
imgObject.src='checked.gif';
radioObject.checked=true;
} else {
imgObject.src='unchecked.gif';
radioObject.checked=false;
}
}