It's not a hard 'rule' but it is frowned upon. Calm has also uploaded other works of his own and there are a number of images on the site posted by their own creators.
Personally, I think it seems a bit presumptuous to post your own work.
Calm said: I wasn't aware of that, but consider it noted. I'll stop posting stuff here, I guess. :)
I don't have a problem, just noticed that you were uploading stuff you created. xD It's well done, don't want you getting banned or anything like that for doing so.
I guess I'm just too used to the sites where uploading your own stuff is the norm, and uploading other people's stuff is considered a violation of copyright. :)
I also agree that this picture doesn't entirely fit in on this site because of its style. So if it gets removed, I'll understand.