I wondered this myself and got to this post. I solved it and figured i would share the my solution.
var imageData = Convert.FromBase64String("/9j/4AAQSkZJRgABAgEASABIAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEB");
var contentId = Guid.NewGuid().ToString();
var linkedResource = new LinkedResource(new MemoryStream(imageData), "image/jpeg");
linkedResource.ContentId = contentId;
linkedResource.TransferEncoding = TransferEncoding.Base64;
var body = string.Format("<img src=\"cid:{0}\" />", contentId);
var htmlView = AlternateView.CreateAlternateViewFromString(body, null, "text/html");
htmlView.LinkedResources.Add(linkedResource);