Upload image with alert message

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ASP.NET

C#3.5

Dear Friends,

I have placed some design & code of my example. Please go throw these.

Design is:

Select Your State : <asp:DropDownList ID="ddlState" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ddlState_SelectedIndexChanged"
Height="35px" Width="325px">



Select your City : <asp:DropDownList ID="ddlCity" runat="server" AutoPostBack="True" Height="35px" Width="325px">

select your Image : <asp:FileUpload ID="FileUpload1" runat="server" />

Enter your Email : <asp:TextBox ID="txtEmail" runat="server" Height="30px" Width="320px">

Enter your Mobile : <asp:TextBox ID="txtMobile" runat="server" Height="30px" Width="320px">

<asp:Button ID="btnSubmit" runat="server" onclick="btnSubmit_Click"
Text="Submit" />

<asp:ValidationSummary ID="ValidationSummary1" runat="server" ShowMessageBox="True"
ShowSummary="False" />


C# Code behind as:

protected void btnSubmit_Click(object sender, EventArgs e)
    {
        if (string.IsNullOrEmpty(txtEmail.Text) && string.IsNullOrEmpty(txtMobile.Text))
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('please enter your email or mobile.')", true);
        }
        else
        {

            if (FileUpload1.PostedFile.ContentLength != 0)
            {
                string strimage = @"~\Images\" + FileUpload1.FileName;
                FileUpload1.PostedFile.SaveAs(Server.MapPath(strimage));
                String strInsert = "insert into Table1(State,City,Image,Email,Mobile)values('" + ddlState.SelectedItem.Text + "','" + ddlCity.SelectedItem.Text + "','" + strimage + "', '" + txtEmail.Text + "','" + txtMobile.Text + "')";
                SqlCommand cmd = new SqlCommand(strInsert, con);
                try
                {
                    con.Open();
                    cmd.ExecuteNonQuery();
                    ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('Your form has been submitted Successfully')", true);
                    lblDisplay.Text = "";
                }
                catch (SqlException ex)
                {

                }
                finally
                {
                    con.Close();
                }

            }
        }
    }


Now my problem is when ever user didn't select any mobile & email, it will show error message as 'please enter your email or mobile' on message box. This time i got the major problem, i.e the hole page is refreshed and already selected image also refreshed, this user again must need to chosen the image again & again. I don't want to refresh the selected image even i got error message on mobile & email. 

Please ASAP give me your suggestions. Thanks in advance.