I have an odd experience here. I have had loads of problems embedding a logo or image to emails using the src=cid... found out it wont show if not setting size for example.<
You need to set PR_ATTACH_CONTENT_ID property (DASL - http://schemas.microsoft.com/mapi/proptag/0x3712001F) on the attachment using Attachment.PropertyAccessor. Be aware, the PropertyAccessor property of the Attachment class was added in Outlook 2007.
You may find the How do I embed image in Outlook Message in VBA? link helpful.