As suggested by the author, try this
[gallery openseadragon="true" zoomimages="/assets/images/seadragon/rhizome_warehouse/dzc_output.dzi" noattachments="true"]
Removing https://www.nathanielmcmahon.com resolves the CORS issue.
If you are still having trouble with it, you could use 3rd party service such as zoomable.ca or zoomo.ca. It’s free and user friendly (upload your image and embed the viewer to your site).
