You can add an image to a node by doing something like this:
onCreateLabel: function(label, node){
label.innerHTML = "<div><img src='/images/node.png'/></div>;
}
To add a unique image you can pass in an image name in json:
onCreateLabel: function(label, node){
label.innerHTML = "<div><img src='/images/"+node.data.image+"'/></div>;
}