as far as i know you can do this in LB only by right click on the specific game, edit/edit metadata/graphics. Here you can import/ add the graphics for the preview; click on below to "ok".
Keep in mind, also to define which kind of graphics this will be (e.g. 3d box etc.). Thats it and then you will see immeadetly the defined graphics for a specific gane. It takes less than a minute (depending on your graphics group).