There really is not a setting in Launchbox/BigBox for that. It might be something that can be altered in a specific theme xml file, but I do not know how to do that. By default BigBox looks in the Arcade-Marquee folder of the system specific image folders for the marquee of each game. If nothing is present there than I believe it defaults to the box image. So if you want certain images for each game place them in the Arcade-Marquee folder. Each file has to be named after the game as it is named in your Launchbox set-up. For now BB only supports image files for marquees.