Concept art and references of Ash, May, Brock and Max drawn by Ken Sugimori for the Advance Generation series of the Pokemon Anime. They were originally posted on Game Freak's webpage.
@PokemonArtFacts
something interesting to note is that May's sketch is labelled "girl hero, roughly 10 years old", but Max's sketch is labelled "Haruka's little brother" (Haruka being May's Japanese name), so it seems that Max was designed later since May didn't have a name back then