I'll tell you since the artist won't. Go to the paint shop in the game. Select 'create vital groups' and a white or black screen should appear with paint tools. Make sure it's a screen and not the car itself. Use it to create the MLP picture when you're done press save and exit the painting. Then go to 'applay vital groups' and there you go.
Or if you're lazy you can buy the designs from someone else on the Playstation, X-box marketplace. You'll have to be signed in to X-box live or the Playstaion network in order to buy the designs.
If you still don't understand what I meant by the designs then there's pletty of tutoials on Youtube that will show you how to make the designs. Hope that helps.