If possible, please don't use fanart on the wiki.
I'm sure there are instances where it would make sense, like for characters or scenes that Mortis didn't specifically draw himself. For most characters and enemies, it should be very easy to get official artwork though. I will be a very sad panda if I see people doing things like replacing character art with fanart just because it looks cooler.
If posting fanart, ideally you need the artist's permission. Every time you upload an image to Wikia (even if you link off-site, it uploads it remotely) you license it under a CC sharealike license. You really need either licensed fanart or the artist's permission to do that. Please at least source the artist. No, not in the article, but on the image page. Unsourced or unlicensed art may be removed.
I realize that the actual concept art isn't licensed, which is why I'm not gonna be super strict with these rules as long as you behave appropriately. Mortis did give recon dragon permission to use and distribute his sketches, so I'm very okay with uploading it here.