The 'rules' for giving upvotes or downvotes here are (not so) simple. If you like the artwork, either people upvote or leave it undisturbed. If you don't like the artwork, people tend to leave a downvote. Granted, there's a kind of herd mentality of giving downvotes: the more downvotes a post has, the higher the chance of people leaving downvotes. Your art being in the comment section doesn't help, since it gets more view than usual.