You can set the ImageUrl and ExpandedImageUrl properties to the URL of images that will show before the node text in a treeview structure.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.tree.textandimages._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en">
<head id="Head1" runat="server">
    <meta charset="utf-8">
    <title>jqTreeView for ASP.NET WebForms - Text And Images</title>
    <!-- The jQuery UI theme that will be used by the treeview  -->
    <link rel="stylesheet" type="text/css" media="screen" href="" />
    <!-- The jQuery UI theme extension jqTreeView needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqtreeview.css" />
    <!-- jQuery runtime minified -->
    <script src="/js/jquery-3.2.1.min.js" type="text/javascript"></script>    
    <!-- The jqTreeView client-side javascript -->
    <script src="/js/trirand/jquery.jqTreeView.min.js" type="text/javascript"></script>    
    <style type="text/css">    
        body, html { font-size: 80%; }    

    <form id="form1" runat="server">
        <trirand:JQTreeView runat="server" ID="JQTreeView1" Width="200" Height="300">
                <trirand:JQTreeNode Text="One" ImageUrl="/images/document.png" />
                <trirand:JQTreeNode Text="Two" ImageUrl="/images/folder.png" ExpandedImageUrl="/images/folder-open.png" Expanded="true">
                        <trirand:JQTreeNode Text="2.1" ImageUrl="/images/document.png" Url="" />
                        <trirand:JQTreeNode Text="2.2" ImageUrl="/images/folder.png" ExpandedImageUrl="/images/folder-open.png" Expanded="true">
                                <trirand:JQTreeNode Text="2.2.1" ImageUrl="/images/document.png"/>
                                <trirand:JQTreeNode Text="2.2.2" ImageUrl="/images/document.png"/>
                                <trirand:JQTreeNode Text="2.2.3" ImageUrl="/images/document.png"/>
                        <trirand:JQTreeNode Text="2.3" ImageUrl="/images/document.png" />
                <trirand:JQTreeNode Text="Three" ImageUrl="/images/document.png" />
        <br />
        <br />
        <trirand:codetabs runat="server" ID="tabs"></trirand:codetabs>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace JQSuiteASPNETExample.examples.tree.textandimages
    public partial class _default : System.Web.UI.Page
        protected void Page_Load(object sender, EventArgs e)

Switch theme:

Theming is based on the very popular jQuery ThemeRoller standard. This is the same theming mechanism used by jQuery UI and is now a de-facto standard for jQuery based components. The benefits of using ThemeRoller are huge. We can offer a big set of ready to use themes created by professional designers, including Windows-like themes (Redmond), Apple-like theme (Cupertino), etc. In addition to that any jQuery UI controls on the same page will pick the same theme.

Last, but not least, you can always roll your own ThemeRoller theme, using the superb Theme Editor

To use a theme, simply reference 2 Css files in your Html <head> section - the ThemeRoller theme you wish to use, and the jqGrid own ThemeRoller Css file. For example (Redmond theme):

<link rel="stylesheet" type="text/css" media="screen" href="/themes/redmond/jquery-ui-1.8.2.custom.css" />
<link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />