This is the reason Japanese artists have such a bad impression of overseas fans.
It’s not harmless – if you don’t have permission, you don’t post – it’s that simple. You know perfectly well a Japanese artist can’t contact you and ask you to take it down, don’t play stupid.
But I can. I will contact the artists whose work you have reposted and ask them to make a statement asking you to take it down.
This goes to everyone reposting art from sites like twitter – please stop. If you want to share it – post a link to the artist’s page.
Of course you shouldn’t repost any art, but at least western artists can contact you on their own behalf – Japanese artists cannot.