Let's say you already have the fundamentals of your game: you created gameplay that you and your (possible) playtesters find enjoyable to play, and you think you can flesh out your game more. However, it's all just colorless blocks and cones at this stage. It's still just a prototype and you don't have a clear vision of how you want the game to actually look.
Before you try to glue together something compelling in your game engine, you want to create a mockup of how your game could possibly look like, just like how Bloodstained did on their Kickstarter page:
These aren't ingame screenshots, those were made before the game even started. How are these prototype images called and what are their artists called? What kind of "keywords" do I have to look for if I want to hire an artist to prototype one or multiple possible looks for my game?