Demonstrating attaching an image to a MailMessage from a url.

MailExample.cs

var url = "https://chart.googleapis.com/chart?chs=250x250&cht=qr&chl=abc%20123%20456&choe=UTF-8";

var attachment = MailMessageHelper.GetUrlAsAttachment(url, "qrcode.png");
message += string.Format("<img src='{0}' />", attachment != null ? "cid:" + attachment.ContentId : url);

using (SmtpClient client = new SmtpClient("xxx.xxx.x.xx"))
{
  MailAddress to = new MailAddress("john@example.com");
  MailAddress from = new MailAddress("email@example.com");
  MailMessage email = new MailMessage(from, to);
  email.Subject = "Subject";
  email.Body = message;
  email.IsBodyHtml = true;
  try
  {
    client.Send(email);
  }
  catch (Exception ex)
  {
    Console.Write(ex);
  }
} 

MailMessageHelper.cs

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

public static class MailMessageHelper
{
  public Attachment GetUrlAsAttachment(string url, string attachmentName) /* using System.Net; */
  {
    try {
      var uri = new Uri(url);
      var request = (HttpWebRequest)WebRequest.Create(uri);
      var response = (HttpWebResponse)request.GetResponse();
      var attachment =  new Attachment(response.GetResponseStream(), attachmentName);
      attachment.ContentDisposition.Inline = true;
      return attachment;
    } catch (UriFormatException uex) {
      // Something wrong with the URL
      return null;
    } catch (WebException wex) {
      // Something wrong with the URL, but according to the server.
      // This could be a 404 (Not Found), 500 (Server error), etc.
      return null;
    } catch (Exception ex) {
      // SOmething unanticipated went wrong.
      return null;
    }
  }
}