Overview
This article introduces how to register IIIF images as media in Omeka S. Images published on external IIIF image servers can be registered in Omeka S.
The official manual describes this at the following page.
https://omeka.org/s/docs/user-manual/content/items/#media
Specific Method
In the media registration screen of the item edit page, enter values as shown below.

You need to enter a URL ending with /info.json. The following uses the National Diet Library’s Koui Genji Monogatari as an example.
https://dl.ndl.go.jp/api/iiif/3437686/R0000001/info.json
As a result, IIIF images can be used from Omeka as shown below.

Supplement
Starting from Omeka S v4, the ability to import external IIIF manifest files is also provided.

https://dl.ndl.go.jp/api/iiif/3437686/manifest.json
Summary
The key point when registering IIIF images is to append /info.json to the end of the URL. I hope this serves as a useful reference when using IIIF with Omeka.