I usually just upload things normally. If it should be labeled as mature, someone may report it. That isn't a bad thing, though. Just because something is reported doesn't mean it will be removed. Everything that is reported is instantly labeled as mature and inspected later on. As long as the aren't against the guidelines, and it doesn't sound like your paintings will be, they will be kept online and seen as mature. In conclusion, I would just upload it normally without the mature tag. If someone reports it, it will be given the tag and that's all. Hope this helps! :3

Thanks! Time to upload publicly! ^u^