Ref:http://www.aspforums.net/Threads/817841/Display-Image-as-ToolTip-on-MouseOver-of-GridView-Row-in-ASPNet/

Display Images from Folder in ASP.Net

Here using the Ajax Hover Menu extender you can show the complete GridViewRow data in Popup.

HTML

<form id="form1" runat="server">
<div>
    <cc1:ToolkitScriptManager runat="server">
    </cc1:ToolkitScriptManager>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" ShowHeader="false">
        <RowStyle Height="40px" />
        <Columns>
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:Panel ID="Panel1" runat="server" Width="400px">
                        <asp:Label ID="Label4" runat="server" Text='<%# Eval("Text") %>'></asp:Label>
                    </asp:Panel>
                    <cc1:HoverMenuExtender ID="HoverMenuExtender1" runat="server" PopupControlID="PopupMenu"
                        TargetControlID="Panel1" PopupPosition="Bottom">
                    </cc1:HoverMenuExtender>
                    <asp:Panel ID="PopupMenu" runat="server" CssClass="modalPopup">
                        <asp:Label ID="Label1" runat="server" Text='<%# Eval("Text") %>'></asp:Label>
                        <br />
                        <hr />
                        <asp:Image ID="Image1" runat="server" CssClass="popupHover" ImageUrl='<%# Eval("Value") %>'
                            Height="100px" Width="100px" />
                    </asp:Panel>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
</form>

Style

<style type="text/css">
    .modalPopup
    {
        background-color: #FFFFFF;
        border-width: 3px;
        border-style: solid;
        border-color: black;
        margin: 10px;
        padding: 10px;
        width: auto;
        height: auto;
    }
    body
    {
        cursor: pointer;
        font-family: Arial;
        font-size: 10pt;
    }
    table
    {
        border: 1px solid #ccc;
    }
    table th
    {
        background-color: #F7F7F7;
        color: #333;
        font-weight: bold;
    }
    table th, table td
    {
        padding: 5px;
        border-color: #ccc;
    }
</style>

Namespaces

using System.IO;
using System.Collections.Generic;

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string[] filePaths = Directory.GetFiles(Server.MapPath("~/Images/"));
        System.Collections.Generic.List<ListItem> files = new List<ListItem>();
        foreach (string filePath in filePaths)
        {
            string fileName = Path.GetFileName(filePath);
            files.Add(new ListItem(fileName, "~/Images/" + fileName));
        }
        GridView1.DataSource = files;
        GridView1.DataBind();
    }
}

Screenshot