I had these as Discord embeds with no source along with post #3624899 and post #3476725. SauceNAO does point to both Pixiv and Twitter posts for these, both 404, and unfortunately the Tweet (https://twitter.com/unimon_e/status/1112916542952431616) isn't on archive.org. This file comes from Gelbooru, and uploading the one I got from Discord here yields a 96% duplicate match.