Hi - so the ARM staff were re-discussing this recently (especially since we now have an image board).
And basically, we've decided to be a lot less strict about this. In our opinion, it's now fine to post fan arts here (and on the image board). We mainly suggest that you credit the images if you know who the artist is, but to not worry about that detail otherwise.
The reason for that change: Basically (like I mentioned in a mini-rant recently in Observatory), our previous visitors (campaigners on behalf of all JP fan-artists) didn't have any standing to demand that all Japanese fanarts be taken down.
They weren't official representatives of the entire Japanese fan artist community, any more than any one of our forum artists could be an official representative of the entire western fan artist community.
And furthermore - those art pieces are already hosted in many other places (eg: Japanese image boards), without any problems, and we haven't heard of any major image boards (like 4chan or danboruu) being shut down, or being requested to remove all their fan art.
Though actually, places like those do get some official take-down requests, like in this case:
But as you can see there, it's a very specific request - Artist A is going around to many websites, and sending legal-sounding notices to get their work taken down. And it's pretty rare too, maybe once every few years that that happens.
However, it's totally unprecedented to be requested for absolutely all art of a given genre (no matter who drew it) to be removed, and there's no legal basis in such a request, no matter how many online manifestos you point to.
So anyhow - we've decided that rather than going out of our way to avoid offending some specific vigilante group, we're going to resume posting fan arts. And if they visit us again later, we'll require them to be more precise about which artists want their art removed, and eg: request a link to that artists blog where they made the request (one of our Japanese speakers can try to contact them back over email, assuming they even bother to reply).
Otherwise, we can pretty much assume that the group is just trolling us, similar to if some of our members made a manifesto and went to Japanese boards and demanded in Japanese for all western-styled art to be taken down, which would hardly be reasonable, either.
Anyhow, we're also revising the rules topic, and after this point fan arts are fine for posting here and on the image board, without any special restrictions (unless you also want to list the artist name for courtesy’s sake, or we've confirmed that a given artist actually doesn't want their fan art re-posted).
edit: Globalled this topic.