Stupid Sailing Newbie question
Anyone with a handle of RC Phd should know everything.
But, I will still answer your question for you.
You put graphics on the sails the same way you would with any other old sailing vessel. You have a couple of old salts climb the sails and paint them with the blood of the hapless captured and killed victims of their their last "booty" raid. Of course.
All other stupid questions must be accompanied by an explanation of why graphics belong on a sailplane that flies so high you can't see them anyway. Hah, so there!!!