If you want to send an HTML email with embedded image please take a look at this article:
https://www.limilabs.com/blog/send-html-email
If you really want to add RTF content to your email use MailBuilder.Rtf
property:
var builder = new MailBuilder();
builder.Subject = "Test email";
builder.From.Add(new MailBox("alice@mail.com", "Alice"));
builder.To.Add(new MailBox("bob@mail.com", "Bob"));
builder.Rtf = @"{\rtf Hello\par This is {\b bold text}.\par}";
IMail email = builder.Create();
There are many articles online about how to create RTF data containing an image.