Grand Admiral Thrawn is the only Imperial officer to get a cool design painted on the bottom of his Star Destroyer. Which means he had to get special permission from somebody like the Emperor to make that happen.
Imagine how that conversation must have gone.
“Yes, my servant, you may trick out your warship, for the custom paint job you have requested is rad as balls. Go forth and make the Rebellion super jelly, in the name of the Empire.”