From what I heard, not exactly. They're the work of a friend of the author who is a professional painter. His paintings have been used by Fujita all through the series, and I believe this painter has had exhibits in Japan.
You can see quite a lot of his work in higher quality on his (no longer active) deviantart account! https://www.deviantart.com/hakota
Fun fact, even those "Son of Takoha" characters that Yokiyo (or whichever) brought to life? Those were from a painting from years earlier that Hakota had done. Not made for the manga. He just animated his own friend's surreal characters.