If your mod calls for an image that the race dosen't have, it will try to use the generic one.
For P&N, I had a "Battlemoon" ship size, and used a large rock/none planet as the generic image.
Races such as the Space Vikings will use thier own picture, but if they don't have a special one, the planet shows up instead.
I've lost a few battlemoons because I mistook them for simple uninhabited planets
