This blogger talks about his memories of HoMM, throughout games I to IV. He touches the topic of the art style well.
IMO the apogee of the art seems to be II, which improves over I. The graphics in III seem to look great in the world map, but in the battle screen they look copy-pasted together, and the background doesn’t really fit. Plus, the shading was softer in I and II, meanwhile in III it is clear that the left side army has shadows in one direction and the other side has shadows in the other.
It seems HoMM II has the same art director, but different artists.
