All images for Platform Categories (Console, Arcade, etc..), Platforms (Arcade, Super Nintendo, Sega Genesis, etc...), Playlists (Favorites, Namco Classics, etc...) will have their own folder structures to follow. If you have already imported games into LaunchBox then it will have already created general structures for what you imported. Just head into your \Launchbox\Images folder and spend some time in it. You will quickly see how the structure is laid out.
Images for the Platforms (The platform name itself not the games for a platform) go in: \Launchbox\Images\Platform\[Platform Specific Name] then there will be sub folders like Banner, Clear Logo, Device, etc.. where you place each unique image.