I would avoid using concept art as it's usually something from the design process of the game and can even be copyrighted by third parties or artists. A screenshot of the game from the developers website should be fine as it's a screenshot of the game, although you don't want something misleading, always double check if they have a press kit too, sometimes they include logos, screenshots and images for use that can make creating thumbnails a little easier. If you're not sure of whether to use the art or not, contact the developers. They are the only 1s who know who the copyrights of the concept art belongs to.