my recollection is if you include files in the same directory as your twine, you can then reference it in the twine itself. so i think it'd require (for posting) uploading it as part of the same directory, which I think itch supports, or via a third-party image source that's linked.
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you can really argue that captian underpants has interactive visual elements, thus it's a narrative game
twine resources are very vague on that
https://twinery.org/forum/discussion/3044/how-to-import-images-into-twine-2
i have four chapters