Solution
While we do support adding attachments to Transactional Messages (https://customer.io/docs/transactional-api-examples/#including-an-attachment), we don’t have any support for referencing images via their Content-ID. As noted in our docs, the Content-ID header cannot be modified/overwritten. (https://customer.io/docs/transactional-faq/#what-headers-are-reserved-and-cannot-be-overwritten-in-the-api-request)
Using Content-ID references is a fairly old practice and one that is generally not recommended for cross-platform multi-client compatibility; in most cases, externally hosted images are the best bet. Another method some people have tried is to use base64 images, but support for this is not great and not something we would recommend. (https://www.caniemail.com/features/image-base64/)
If the images simply vary (e.g. vary between image A, image B, and image C), uploading all of your images and then using something like Liquid in the Dynamic Image field to select the correct image might be the best bet here.