You can add client events for initialize, expand and collapse node like this:

var j = $(".igdt_NodeHolder img:first-child").each(function (a, img) {
var attr = $(img).attr("idx")
if (attr == "1") {
$(img).attr("class", "collapsed");
}
else if (attr == "2") {
$(img).attr("class", "expanded");
}
else {
$(img).attr("class", "leaf");
}

});

Also, add css classes to your stylesheet:

.ig_Control.igdt_Control .igdt_NodeHolder .expanded
{
width:0px;
height:0px;
padding: 5px 5px 6px 6px;
margin-right:4px;
background:url('../Images/icons/collapsenode.png');
}
.ig_Control.igdt_Control .igdt_NodeHolder .collapsed
{
width:0px;
height:0px;
padding: 5px 5px 6px 6px;
margin-right:4px;
background:url('../Images/icons/expandnode.png');
}
.ig_Control.igdt_Control .igdt_NodeHolder .leaf
{
display:none;
}