To see image everywhere you can use these options:
Absolute Url
You can simply use full absolute path of image for example "http://example.com/images/logo.png"
IMO It is the most simple option and recommended for your problem.
Attachment
As mentioned by Mason in comments You can attach image to mail and then put image tag and useContentId of attachment:
//(Thanks to Mason for comment and Thanks to Bartosz Kosarzyck for sample code)
string subject = "Subject";
string body = @"<img src=""$CONTENTID$""/> <br/> Some Content";
MailMessage mail = new MailMessage();
mail.From = new MailAddress("[email protected]");
mail.To.Add(new MailAddress("[email protected]"));
mail.Subject = subject;
mail.Body = body;
mail.Priority = MailPriority.Normal;
string contentID = Guid.NewGuid().ToString().Replace("-", "");
body = body.Replace("$CONTENTID$", "cid:" + contentID);
AlternateView htmlView = AlternateView.CreateAlternateViewFromString(body, null, "text/html");
//path of image or stream
LinkedResource imagelink = new LinkedResource(@"C:UsersR.AghaeiDesktopoutlook.png", "image/png");
imagelink.ContentId = contentID;
imagelink.TransferEncoding = System.Net.Mime.TransferEncoding.Base64;
htmlView.LinkedResources.Add(imagelink);
mail.AlternateViews.Add(htmlView);
SmtpClient client = new SmtpClient();
client.Host = "mail.example.com";
client.Credentials = new NetworkCredential("[email protected]", "password");
client.Send(mail);
Data Uri
you can use data uri (data:image/png;base64,....).
Not Recommended because of weak support in most of mail clients, I tested it with Outlook.com(web) and OutlookWebAccess(web) and Office Outlook(Windows) and Outlook(windows 8.1) and unfortunately it worked only on OutlookWebAccess(web).
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