To give you a detailed answer, I would need to understand more about your expectation. However, at a high level, you can have an email sent from Adobe Campaign that references images that are stored on another server, for example, a digital asset management system.
We have completed work for our clients that have simply been to build an HTML email for Adobe Campaign where all images are hosted by a third party system.
We have also done work for clients; when they want to store and display those External images in a form and bring them into an email dynamically.
With further information - I may be able to help you further but does that at least provide you with an answer?