If one of the predefined cell formatters (integer, link, email, number, currency, checkbox) is not suitable in your scenario, you can use custom formatters.

With custom formatters you can define your own javascript client-side format and unformat functions. Format function are used to display the cell and unformat is used to unformat the cell in display mode and show it in edit mode.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="GridTest.examples.functionality.custom_formatter._default" %>

<!DOCTYPE html>
<html lang="en-us">
<head id="Head1" runat="server">
    <title>jqGrid for ASP.NET WebForms - custom cell formatter</title>
    <!-- The jQuery UI theme that will be used by the grid -->
    <link rel="stylesheet" type="text/css" media="screen" href="http://ajax.aspnetcdn.com/ajax/jquery.ui/1.11.4/themes/redmond/jquery-ui.css" />
    <!-- The jQuery UI theme extension jqGrid needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />
    <!-- jQuery runtime minified -->
    <script src="http://ajax.microsoft.com/ajax/jquery/jquery-2.2.0.min.js" type="text/javascript"></script>
    <!-- The localization file we need, English in this case -->
    <script src="/js/trirand/i18n/grid.locale-en.js" type="text/javascript"></script>
    <!-- The jqGrid client-side javascript -->
    <script src="/js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script>    
    
    <style type="text/css">
        body, html { font-size: 80%; }    
    </style>
</head>
<body>
    <form id="form1" runat="server">
    <div>       
       <trirand:JQGrid ID="JQGrid1" runat="server" Width="400px" Height="350">
            <Columns>
                <trirand:JQGridColumn DataField="ID" Width="50"/>
                <trirand:JQGridColumn DataField="PhotoFileName" Width="100" />
                <trirand:JQGridColumn DataField="Photo" Width="100">
                    <Formatter>
                        <trirand:CustomFormatter FormatFunction="formatImage" UnFormatFunction="unformatImage" />
                    </Formatter>
                </trirand:JQGridColumn>  
                <trirand:JQGridColumn DataField="Rating" TextAlign="Center">
                    <Formatter>
                        <trirand:CustomFormatter FormatFunction="formatImage" UnFormatFunction="unformatRating" />
                    </Formatter>
                </trirand:JQGridColumn>  
            </Columns>            
       </trirand:JQGrid>
       
       <script type="text/javascript">
           // The FormatFunction for CustomFormatter gets three parameters           
           // cellValue - the original value of the cell
           // options - as set of options, e.g
           //   options.rowId - the primary key of the row
           //   options.colModel - colModel of the column
           // rowData - array of cell data for the row, so you can access other cells in the row if needed
           
           function formatImage(cellValue, options, rowObject) {
               var imageHtml = "<img src='images/" + cellValue + "' originalValue='" + cellValue + "' />";
               return imageHtml;
           }

           // The FormatFunction for CustomFormatter gets three parameters           
           // cellValue - the original value of the cell
           // options - as set of options, e.g
           //   options.rowId - the primary key of the row
           //   options.colModel - colModel of the column
           // cellObject - the HMTL of the cell (td) holding the actual value
           function unformatImage(cellValue, options, cellObject) {
               return $(cellObject.html()).attr("originalValue");
           }

           function formatRating(cellValue, options, rowObject) {
               var color = (parseInt(cellValue) > 0) ? "green" : "red";
               var cellHtml = "<span style='color:" + color + "' originalValue='" + 
                                    cellValue + "'>" + cellValue + "</span>";
               
               return cellHtml;
           }

           function unformatRating(cellValue, options, cellObject) {
               return $(cellObject.html()).attr("originalValue");
           }
       </script>
       
       <br /><br />
       <trirand:codetabs runat="server" id="DataTableCodeTabs"></trirand:codetabs>    
       
    </div>
    </form>
</body>
</html>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using Trirand.Web.UI.WebControls;

namespace GridTest.examples.functionality.custom_formatter
{
    public partial class _default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (JQGrid1.AjaxCallBackMode == AjaxCallBackMode.RequestData)
            {
                JQGrid1.DataSource = GetData();
                JQGrid1.DataBind();
            }
        }

        private DataTable GetData()
        {
            DataTable dt = new DataTable();
            dt.Columns.Add(new DataColumn("ID", typeof(int)));
            dt.Columns.Add(new DataColumn("PhotoFileName", typeof(string)));
            dt.Columns.Add(new DataColumn("Photo", typeof(string)));
            dt.Columns.Add(new DataColumn("Rating", typeof(string)));

            dt.Rows.Add(new object[] { "1", "1.jpg", "1.jpg", 20 });
            dt.Rows.Add(new object[] { "2", "2.jpg", "2.jpg", -1 });
            dt.Rows.Add(new object[] { "3", "3.jpg", "3.jpg", -15 });
            dt.Rows.Add(new object[] { "4", "4.jpg", "4.jpg", 23 });
            dt.Rows.Add(new object[] { "5", "5.jpg", "5.jpg", -21 });
            dt.Rows.Add(new object[] { "6", "6.jpg", "6.jpg", 2});
            dt.Rows.Add(new object[] { "7", "7.jpg", "7.jpg", 6 });
            dt.Rows.Add(new object[] { "8", "8.jpg", "8.jpg", 11 });
            dt.Rows.Add(new object[] { "9", "9.jpg", "9.jpg", -31 });

            return dt;
        }
    }
}
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" />