Right now, the image is visible on the page but trying to access it with no Referer header fails with a 403 error. Isn’t that what you’re trying to accomplish? It’s working.
Be aware that legitimate browser access within the context of the page won’t always have a Referer header, so you will be blocking some legitimate views of the image by doing this. And that anyone can manually retrieve your image directly by simply adding a Referer header.
% curl -H 'Referer: https://mangas-origines.fr/' -I https://mangas-origines.fr/wp-content/uploads/WP-manga/data/manga_6335704bc33d7/4e1c94e81a40a33d98468e4db4a019b9/page_0.jpg
HTTP/2 200
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