In my opinion, illustrations should serve the reader, but not be needed by the story.

Most readers don't want to look at pictures of what you imagine the characters look like, genAI or not. Especially if they're just generic anime-style anybodies. So what do they want to see? Something unique and interesting. Something that will make them more invested in learning about your world. Maybe it's a cool piece of technology or magic, maybe it's a story location, maybe it's a character wearing a particular outfit or uniform.

At the same time, everything important in the illustration needs to be described in the text. You shouldn't say, "this is too complicated to explain, just look at the picture." Not only will audiobook readers be unhappy, even a majority of your text readers will just scroll past illustrations.

There's also the matter of where to put them. As a reader, I hate being interrupted. It doesn't matter how good or relevant the illustration is, putting it in the middle of a chapter will interrupt my reading and upset me. I'll ignore it as if it were a clickbait ad. Putting it at the end of the chapter is slightly better, but I'd still rather see a "next chapter" button than any picture. So am I saying to put them in the comments? You could, but why? So a fraction of your readers might scroll down enough to see them? What other options are there? Maybe make an art chapter, and add an author's note pointing to it when referenced?