Try putting your tree view in a div and set the background image for the div.
If the above doesn't help try putting the following inside the head tag of your page:
------------------------------------------------------------------------------------
<style type="text/css">
.treeBgImg{background-image : url(<yourimageurl>);}
</style>
<script type="text/javascript">
window.onload = function(){
document.getElementById("<%=myTreeView.ClientID%>").className = "treeBgImg";
//specify your treeview id instead of myTreeView
}
</script>
---------------------------------------------------------------------------
Hope this helps.