Use MailBuilder.LoadHtml method. It sets HTML version of the message. Modifies passed HTML, so that img tags reference inline images. Adds all images that can be found on disk to Visuals collection. Extracts plain text from HTML and sets IMail.Text property.
MailBuilder builder = new MailBuilder();
builder.LoadHtml(@"<img src=""c:\BRHB00001256/IS20KG.bmp"" align=baseline>");
Assert.AreEqual(1, builder.Visuals.Count);
Assert.AreEqual("IS20KG.bmp", ((MimeData)builder.Visuals[0]).FileName);
StringAssert.IsMatch(@"\<img src=\""cid\:(.+)@mail\.dll\"".*", builder.Html);