If you want to embedded in to body use
try
{
Attachment oAttachment = oMail.AddAttachment( "d:\\test.gif" );
string contentID = "test001@host";
oAttachment.ContentID = contentID;
oMail.HtmlBody = "<html><body>this is a <img src="\"cid:"<br" mode="hold" /> + contentID + "\"> embedded image.</body></html>";
Console.WriteLine("start to send email with embedded image...");
oSmtp.SendMail(oServer, oMail);
Console.WriteLine("email was sent successfully!");
}
catch (Exception ep)
{
Console.WriteLine("failed to send email with the following error:");
Console.WriteLine(ep.Message);
}