I started using LaunchBox recently and I must say that it is amazing. I will upgrade to the Premium version even if I don't need it just to support you, guys.
I arrived here because I was about to ask the same question as NicoDirkfan. IMHO, if a user selects Image Group --> 3D Box, and a specific game doesn't have one, the image should default to another one. I guess that the user can just copy the front box image to the 3D folder, but I didn't try that yet.