silverstripe-cms/templates/Includes/ReportAdmin_left.ss

<style>
	ul.tree a{
		background-image: url(cms/images/treeicons/reports-file.png);
	}
</style>

<h2><% _t('REPORTS','Reports') %></h2>

<div id="treepanes">
<div id="sitetree_holder">
	<% include ReportAdmin_SiteTree %>
</div>
</div>