Augh, I wouldn't mind so much if this other Steam guy was using the right cover art instead of random box shots!
Edit: By the way, the style guide should mention the following:
Games on Steam may not have an ESRB rating. Many games, especially indies, rarely have one. If that's the case then set the rating to "None".
The artwork should be the header.jpg shown in the store page. You can get the full version of this image by visiting this address:
http://cdn.steampowered.com/v/gfx/apps/XXXX/header.jpg where XXXX is the game's Steam app number visible on the browser address.