I probably should've made a formal announcement about this a long while ago, sorry this is so late - All wiki members can upload fanart to Discussion posts, since it won't be conflicting with the rest of the wiki. The images that are uploaded to Discussion posts don't appear in the image namespace. This means if you upload an image to a Discussion post, it won't appear under Special:Images. Only official artwork should be uploaded to the Special:Images namespace. If you want to upload fanart to a Discussion post, just keep basic image guidelines in mind (don't upload anything inappropriate, that kind of thing.)

Again, sorry this is a very late announcement, I just wanted to make sure anyone who wants to share fanart is aware of this option.