This series of tutorials teach about making good design paintings for the entertainment inustries, both in the subject and presenting. I highly recommend you look at it. I haven't been through them all as I'm currently on episode 7 (and each episode is about 2-4 youtube videos) but so far it has already inflluenced me.
The guy who makes them is called Feng Zhu (youtube channel FZDSCHOOL) and you can see the first tutorial embedded below:
Tutorial part 2
There are many more in this series, and I recommend taking a look even if you just do ordinary painting, as some of the things he talks about is applicable to various things.