Use this code and let me know
protected void yourButton_Click(object sender, EventArgs e)
{
string strMailContent = "Welcome new user";
string fromAddress = "yourname@yoursite.com";
string toAddress = "newuser@hisdomain.com";
string contentId = "image1";
string path = Server.MapPath(@"images/Logo.jpg"); // my logo is placed in images folder
MailMessage mailMessage = new MailMessage( fromAddress, toAddress );
mailMessage.Bcc.Add("inkrajesh@hotmail.com"); // put your id here
mailMessage.Subject = "Welcome new User";
LinkedResource logo = new LinkedResource(path);
logo.ContentId = "companylogo";
// done HTML formatting in the next line to display my logo
AlternateView av1 = AlternateView.CreateAlternateViewFromString("<html><body><img src=cid:companylogo/><br></body></html>" + strMailContent, null, MediaTypeNames.Text.Html);
av1.LinkedResources.Add(logo);
mailMessage.AlternateViews.Add(av1);
mailMessage.IsBodyHtml = true;
SmtpClient mailSender = new SmtpClient("localhost"); //use this if you are in the development server
mailSender.Send(mailMessage);
}