I don't see the need of duplicating data.
I think this can be interpreted in a different key as well: pages created from templates are supposed to be completely independent (from the templates) after creation and live their own life, not depending on the template anymore (and not having any connection to the template). Also, the idea of a template is that it's only a starting point for a document, and that the document can be modified in any way after (for images this would mean: add extra images, remove images, add new versions of the images, etc). This independence also says that a template should be deletable at any point and the pages created from that template will remain perfectly functional.
IMO, to keep this independence of pages created from templates, by default the behaviour should be to copy the images (just like all other metadata is), and if the user wants to use only one set of images stored in a different page, they can create that separate page (probably different from the template) and put absolute references to it in the template.
The mechanism more adapted for "centralizing" the control over the display of a set of pages is the mechanism of sheets (using a sheet for a set of documents) rather than templates, for which there is cohesion on all the points I described above and indeed, in that case, the approach would be to keep the images in a single place.