Hi RichardSa,

You can't display loading image inside button.

You have to display as modal and change the button text.

Add an gif image.

Create javascript methods to show image.

OnClientClick of button call Show Image javascript method to display image.

HTML

<style type="text/css">
    .modal {
        position: fixed;
        top: 0;
        left: 0;
        background-color: black;
        z-index: 99;
        opacity: 0.8;
        filter: alpha(opacity=80);
        -moz-opacity: 0.8;
        min-height: 100%;
        width: 100%;
    }

    .loading {
        font-family: Arial;
        font-size: 10pt;
        border: 5px solid #67CFF5;
        width: 300px;
        height: 150px;
        display: none;
        position: fixed;
        background-color: White;
        z-index: 999;
    }
</style>
<script type="text/javascript">
    function disableBtn(btnID, newText) {
        var btn = document.getElementById(btnID);
        btn.disabled = true;
        btn.value = newText;
        ShowProgress();
    }

    function ShowProgress() {
        document.getElementsByClassName("loading")[0].style.display = 'block';
    }
</script>
<div class="loading" align="center">
    Loading. Please wait.<br />
    <br />
    <img src="loading.gif" alt="" />
</div>
Name:<asp:TextBox runat="server" ID="txtName" /><br />
Last:
<asp:TextBox runat="server" ID="textLast" /><br />
Email:
<asp:TextBox runat="server" ID="textemail" /><br />
Number:
<asp:TextBox runat="server" ID="textNumber" /><br />
<asp:Button ID="btnSubmit" Text="Send" runat="server" OnClick="btnSubmit_Click"
    OnClientClick="disableBtn(this.id, 'Sending Email...')" UseSubmitBehavior="false" />
<asp:Label ID="lblMessage" runat="server" /

Namespaces

C#

using System.Configuration;
using System.Net;
using System.Net.Configuration;
using System.Net.Mail;

VB.Net

Imports System.Configuration
Imports System.Net
Imports System.Net.Configuration
Imports System.Net.Mail

Code

C#

protected void btnSubmit_Click(object sender, EventArgs e)
{
    if (txtName.Text != "" & textLast.Text != "" & textNumber.Text != "" & textemail.Text != "")
    {
        SmtpSection smtpSection = (SmtpSection)ConfigurationManager.GetSection("system.net/mailSettings/smtp");
        using (MailMessage mm = new MailMessage(smtpSection.From, "cletusdestiny33@gmail.com"))
        {
            mm.Subject = "FOLLOW UP";
            mm.Body = "Name: " + txtName.Text + " " + textLast.Text + "<br />Email: " + textemail.Text + "<br />MOBILE NUMBER: " + textNumber.Text;
            mm.IsBodyHtml = true;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = smtpSection.Network.Host;
            smtp.EnableSsl = smtpSection.Network.EnableSsl;
            NetworkCredential networkCred = new NetworkCredential(smtpSection.Network.UserName, smtpSection.Network.Password);
            smtp.UseDefaultCredentials = smtpSection.Network.DefaultCredentials;
            smtp.Credentials = networkCred;
            smtp.Port = smtpSection.Network.Port;
            smtp.Send(mm);
            ClientScript.RegisterStartupScript(this.GetType(), "", "alert('Email send succefully.')", true);
        }
    }
    else
    {
       lblMessage.Visible = true;
       lblMessage.ForeColor = System.Drawing.Color.Red;
       lblMessage.Text = "*All Fields Are Required*";
    }
}

VB.Net

Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As EventArgs)
    If txtName.Text <> "" And textLast.Text <> "" And textNumber.Text <> "" And textemail.Text <> "" Then
        Dim smtpSection As SmtpSection = CType(ConfigurationManager.GetSection("system.net/mailSettings/smtp"), SmtpSection)

        Using mm As MailMessage = New MailMessage(smtpSection.From, "cletusdestiny33@gmail.com")
            mm.Subject = "FOLLOW UP"
            mm.Body = "Name: " & txtName.Text & " " + textLast.Text & "<br />Email: " + textemail.Text & "<br />MOBILE NUMBER: " + textNumber.Text
            mm.IsBodyHtml = True
            Dim smtp As SmtpClient = New SmtpClient()
            smtp.Host = smtpSection.Network.Host
            smtp.EnableSsl = smtpSection.Network.EnableSsl
            Dim networkCred As NetworkCredential = New NetworkCredential(smtpSection.Network.UserName, smtpSection.Network.Password)
            smtp.UseDefaultCredentials = smtpSection.Network.DefaultCredentials
            smtp.Credentials = networkCred
            smtp.Port = smtpSection.Network.Port
            smtp.Send(mm)
            ClientScript.RegisterStartupScript(Me.GetType(), "", "alert('Email send succefully.')", True)
        End Using
    Else
        lblMessage.Visible = True
        lblMessage.ForeColor = System.Drawing.Color.Red
        lblMessage.Text = "*All Fields Are Required*"
    End If
End Sub

Screenshot