Build a creative email template within Outlook

BrandPointZero is a creative communications agency. We have been asked by a corporate client to look into creating a series of email templates, with visuals and graphics, that they can use for internal communication. So, the templates would need to be uploaded into their existing email system (Outlook - we are waiting to find out which version). We have capability to design the layout, create images in any format (JPEG, PING etc) and code in HTML, but then are stuck as to how these could be uploaded/embedded into their Outlook system as a template that can be simply opened up, and populated by a non-technical user - with text that doesn't really need to be formatted, links to film hosted on YouTube or Intranet, links to audio files hosted on internal and external platforms etc. We are assuming we would either need to send the right type of file to their IT department so they can do this, or we send it directly to the user who can upload the template onto their own system herself. Is HTML the right format for this? Or do we need to supply the images in a different format so they can be uploaded at the company? How do we make sure the emails appear in the way they have been designed? We'd also need the graphics and images to show automatically i.e. users don't have to 'download pictures' to see the creatively laid out email messages in all their glory - like you do when you receive HTML emails from third party servers.