I believe you can use Video Game Designer as a title for this type of job.
You can also click on the "Find a freelancer" search box, and change it to "Find job" from the drop down, and then enter video game related search terms to see how other clients are listing similar jobs. Hope that helps!
Nina, a video game designer is not someone who makes graphic design for a game. It's a different role.
Raven, you can post for a graphic designer for a video game.