Thumbnails of your maps in size ideally fitting into 1000 x 1000 px with the same ID as used in the spreadsheet should be provided or linked from the spreadsheet.

In case your maps are available via an image service (such as IIIF or Zoomify, Aware, MrSID, IIPImage, etc) then thumbnails are generated automatically.

Institutions using the https://www.georeferencer.com/  can skip this step.