how to apply background image to email

MailMessage mailMessage = new MailMessage();
mailMessage.From = new MailAddress("venkat@gmail.com");
mailMessage.To.Add(new MailAddress("msdotnettechies@gmail.com"));
mailMessage.CC.Add(new MailAddress("user1@gmail.com"));
mailMessage.Bcc.Add(new MailAddress("user2@gmail.com"));
mailMessage.Subject = "Email Checking Asynchronously";
mailMessage.Body = "Email test asynchronous";
mailMessage.IsBodyHtml = true;//to send mail in html or not

SmtpClient smtpClient = new SmtpClient();//portno here
smtpClient.Host = "smtp.gmail.com";
smtpClient.EnableSsl = true ; //True or False depends on SSL Require or not
smtpClient.Credentials = new NetworkCredential("yourGmail@gmail.com", "yourpassword");
//smtpClient.UseDefaultCredentials = true; //true or false depends on you want to default credentials or not
Object mailState = mailMessage;

//this code adds event handler to notify that mail is sent or not
smtpClient.SendCompleted += new SendCompletedEventHandler(smtpClient_SendCompleted);
try
{
smtpClient.SendAsync(mailMessage, mailState);
}
catch (Exception ex)
{
Response.Write(ex.Message);
Response.Write(ex.StackTrace);
}

// this is the event called on main method
void smtpClient_SendCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
{
MailMessage mailMessage = e.UserState as MailMessage;
if (e.Cancelled || e.Error != null)
{

Response.Write(e.Error.Message);
Response.Write(e.Error.StackTrace);
}
else
{
Response.Write("Email sent successfully");
}
}