Dim attach1 As new System.Net.Mail.Attachment( ProjectPath & "images\1.jpg", System.Net.Mime.MediaTypeNames.Image.Jpeg)
message.Attachments.Add(attach1)
Dim pic1 As String = attach1.ContentId
Dim body As String = "<html><body>内容</body></html>"
Dim content As String = "12345678<font size=7 color=red>12345678</font><img src=""cid:"& pic1 &"""/><p>来自张三.</p>"
message.Body = body.Replace("内容", content)
测试没问题