[Andrés]

The Stellar Manipulation window and the ships shown in the left side of the screen during game setup.

Any missing pictures will be replaced with the "neutral" shipset during the game.
It works this way for all vehicle and group pictures, shields and explosions. It doesn't work for pop pics, and I'm not sure what happens with main and race portrait.
But you will have an error in the setup screen if you don't have the pics it shows there, that is race portrait, main, and portrait_battlecruiser.