So it turns out omc.terranetz.jp/sample/0853de…
belongs to a 小雀 (Kogara). I found this pretty much by accident and was able to verify it with cincia.nido.jp/illust_04.html
(Do be wary when checking out the rest of that site).
Still, this is somewhat baffling. The site is live as of this writing, and there is no robots.txt, so why didn't Google or any image search engine ever pick this up?