Hi kankon

You have taken the ImageButton, so you need to typecast the sender as ImageButton.

Please refer below sample.

Database

This Sample makes use of a table named tblFiles whose schema is defined as follows.

Note: You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<form id="form1" runat="server">
    <div>
        <link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" />
        <asp:DataList ID="dlCustomers" runat="server" RepeatColumns="6" CellSpacing="0" RepeatLayout="Table">
            <ItemTemplate>
                <table class="table">
                    <tr>
                        <th colspan="2">
                            <b>
                                <%# Eval("Name") %></b>
                        </th>
                    </tr>
                    <tr>
                        <td colspan="2">
                            <asp:Label ID="lblId" Text='<%# Eval("id") %>' runat="server" />,
                    <%# Eval("ContentType") %>
                        </td>
                    </tr>
                    <tr>
                        <td>Image:
                        </td>
                        <td>
                            <asp:ImageButton ID="imgImage" runat="server" ImageUrl='<%# "data:image/jpg;base64," + Convert.ToBase64String((byte[])Eval("Data")) %>'
                                Height="75px" Width="75px" OnClick="imgImage_Click" />
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2">Name:
                    <%# Eval("Name")%>
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2">ContentType:
                    <%# Eval("ContentType")%>
                        </td>
                    </tr>
                </table>
            </ItemTemplate>
        </asp:DataList>
    </div>
</form>

Namespaces

C#

using System.Data;
using System.Configuration;
using System.Data.SqlClient;

VB.Net 

Imports System.Data
Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

CS

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = this.GetData();
        dlCustomers.DataSource = dt;
        dlCustomers.DataBind();
    }
}

protected void imgImage_Click(object sender, EventArgs e)
{
    string url = "Default.aspx?Id=" + ((sender as ImageButton).NamingContainer.FindControl("lblId") as Label).Text;
    string script = "window.open('" + url + "', 'popup_window', 'width=300,height=100,left=100,top=100,resizable=yes');";
    ClientScript.RegisterStartupScript(this.GetType(), "script", script, true);
}

private DataTable GetData()
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conString))
    {

        using (SqlCommand cmd = new SqlCommand("SELECT Id,Name,ContentType,Data FROM tblFiles WHERE ContentType = 'image/jpeg'"))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    return dt;
                }
            }
        }
    }
}

Default

protected void Page_Load(object sender, EventArgs e)
{
    if (!string.IsNullOrEmpty(Request.QueryString["Id"]))
    {
        Response.Write("Id is : <b>" + Request.QueryString["Id"] + "</b>");
    }
}

VB.Net

VB

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As DataTable = Me.GetData()
        dlCustomers.DataSource = dt
        dlCustomers.DataBind()
    End If
End Sub

Protected Sub imgImage_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim url As String = "Default.aspx?Id=" & (TryCast((TryCast(sender, ImageButton)).NamingContainer.FindControl("lblId"), Label)).Text
    Dim script As String = "window.open('" & url & "', 'popup_window', 'width=300,height=100,left=100,top=100,resizable=yes');"
    ClientScript.RegisterStartupScript(Me.[GetType](), "script", script, True)
End Sub

Private Function GetData() As DataTable
    Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(conString)
        Using cmd As SqlCommand = New SqlCommand("SELECT Id,Name,ContentType,Data FROM tblFiles WHERE ContentType = 'image/jpeg'")
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    Return dt
                End Using
            End Using
        End Using
    End Using
End Function

 Default

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not String.IsNullOrEmpty(Request.QueryString("Id")) Then
        Response.Write("Id is : <b>" & Request.QueryString("Id") & "</b>")
    End If
End Sub

Screenshot