What is a general word for one who creates an image, either by painting, drawing or other forms of illustrating?

You mention a writer but your question relates to imagery. I therefore take your question to refer to the illustrative part of things that are written.

I suggest that you have defined a niche of meaning between artist and illustrator so narrow that there is nothing to fill it. Consider a couple of relevant dictionary definitions for each word:

illustrator =

a person who draws pictures, especially for books

Cambridge dictionary

A person who draws or creates pictures for magazines, books, advertising, etc

lexico

artist =

someone who paints, draws, or makes sculptures

Cambridge dictionary

one who professes and practices an imaginative art

Merriam webster

If we forget the sculptural aspect (which can hardly be relevant to written works ) and look for the extra connotations of artist that are not present in illustrator we are left with a very small area of meaning associated with imagination (or perhaps creativity?). But an illustrator of books, whether a novel, a comic, a graphic novel or other genres, is necessarily imaginative (and creative). Consequently I can find no word in my vocabulary to answer your need; I suggest illustrator remains the best word to use.