A solution to solve this issue is to hide the original icon and use 'formatter' to add an icon to the left of tree field.
<style>
.tree-icon{
display:none;
}
</style>
$('#tg').treegrid({
columns:[[
{field:'name',title:'Name',width:150,
formatter:function(value){
return '<img src="..." style="width:16px;height:18px;vertical-align:bottom"/>'+value;
}
}
]]
});