From what I can tell, you are not setting the content id properly. Try to change the code to the following:
attc.PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/proptag/0x370E001F", "image/bmp");
attc.PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/proptag/0x3712001F", "Bitmap1.bmp@EMBED");
I have done this before a little differently. I embedded images in some emails that I had to send from my web app using an 'alternate view' in the system.net.mail
System.Net.Mail.LinkedResource theContent = new System.Net.Mail.LinkedResource({path to image});
theContent.ContentID = "TheContent";
String altViewString = anEmail.Body.replace("{original imageSource i.e. '../Images/someimage.gif'}","cid:TheContent");
System.Net.Mail.AlternateView altView = System.Net.Mail.AlternateView.CreateAlternateViewFromString(altViewString, Nothing, System.Net.Mime.MediaTypeNames.Text.Html);
altView.LinkedResources.add(theContent);
anEmail.Message.AlternateViews.Add(altView);
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