could you clarify what do you mean by Content ID. There is no such a field with this name in Kentico.
You can either attach files to an email template through the Attachments button (1) or as part of the body (2).
1) Attachment behaves as direct upload only. No reference to content tree.
2) Email template body supports macro syntax. So you can refer any image from the content tree or media library.
Content tree macro example:
You can also create custom reusable macro to simplify your syntax.
Please see also another thread