How do I add my logo at the top of my email message?

Here is my code to send email to user. I want to place the logo.

using (MailMessage mm = new MailMessage("support@mywebsite.com", mailtxtbx.Text))
{
    mm.Subject = "Activate Account";
    string body = "Hello " + txtname.Text.Trim() + ",";
    body += "<br /><br />Successful Registration";
    body += "<br /><br />Please click the button to activate your account";
    body += "<br /><br /><a style='display: block; width: 200px; height: 30px; background: #32CD32;padding: 15px;font-family: Nunito; text-align:center; border-radius: 5px;color: white;font-weight: 700;text-decoration: none;' href = '"
    + Request.Url.AbsoluteUri.Replace("signup.aspx", "activation.aspx?ActCode=" + actCode) + "'>Activate your new account</a>";
    body += "<br /><br />Thanks";
    mm.Body = body;
    mm.IsBodyHtml = true;
 
    SmtpClient SMTP = new SmtpClient("relay-hosting.secureserver.net", 25);
    SMTP.UseDefaultCredentials = false;
    SMTP.Credentials = new NetworkCredential()
    {
        UserName = "support@mywebsite.com",
        Password = "xxxxxxxxxx"
    };
    SMTP.EnableSsl = false;
    SMTP.Send(mm);
}