I work with a few artists for cover and website content with my Players of the Game series. Mikhail Palamarchuk is especially good, and I've featured a few of his finished works in the past.
I'll provide him a description of a character along with images that match the vibe of what I'm wanting.
Here's a great example with Violet. She's a Skin Bot, an android-type character who appears in the upcoming New Players novel. First, I get a rough sketch: