Hi,
I'm wondering this myself.
What I have found is that if you go to the edit and metadata for the single game and then go to images, you can actually select one of the images you do have and change the image type for that game.
So for example if you only have a game screenshot but your other games are 3D boxes then you can change the screenshot to show as a 3D box. Not ideal but it does work.
I do have my concerns about whether this will have any impact when I have do my BigBox setup though.