I believe the image satisfies WP:NFCCP#8 for the following reasons:
As, per WP:NFC#CS, the item is the subject of sourced commentary in the article.
Such commentary includes passages relating to the design of the poster, including that it was intended to "draw your attention" and be "thought-provoking". It is not possible for readers to judge for themselves whether or not the design achieves these intentions without the image of the poster itself being present. Hence I believe it's inclusion "significantly increase readers' understanding of the topic".
Additionally, the commentary mentions that elements of the design (the use of triangles, diamonds and feathers) are traditional Paraguayan motifs. However, as the current rationale says, this "could not practically be conveyed by words alone"; their size, colour, prominence, number, emotive purpose and so on - it is impossible to convey to readers an understanding, with just text, exactly how such elements are utilised within the design and the effectiveness of their use is ultimately up to the reader's interpretation. It is much simpler, quicker and truer to reality for the poster itself to do this. Hence I believe the image's "omission would be detrimental" to the reader's understanding of the design's nature.
The commentary's purpose here is to enhance the reader's understanding of the origins of the poster's design elements, not to substitute for it.
Therefore, overall, I believe it is only the inclusion of the image that allows the reader to truly identify with the object of discussion.
The uploader, TurboGUY (talk) 03:22, 29 November 2019 (UTC)