Danbooru-based image board with a specialization in high-quality images.
cirno fishnets nopan thighhighs touhou ushiki_yoshitaka wallpaper

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I don't know the original source, but she barely resembles Cirno... Even though, this picture is tagged as Cirno on other booru sites too, I'm not convinced about it. Strange, because other artworks of Ushiki Yoshitaka are unmistakable.