string attachmentPath = Environment.CurrentDirectory + @"	est.png";
    Attachment inline = new Attachment(attachmentPath);
    inline.ContentDisposition.Inline = true;
    inline.ContentDisposition.DispositionType = DispositionTypeNames.Inline;
    inline.ContentId = contentID;
    inline.ContentType.MediaType = "image/png";
    inline.ContentType.Name = Path.GetFileName(attachmentPath);

    message.Attachments.Add(inline);

参考:用 C# 发送带有内联附件的电子邮件