Do you mean portraits or icons? Serpent just told you about icons, ask in modelling about portraits I would say.
WHATS THE DIFFERENCE? (Just so you know)
Icons are the 2d art shown in the command bar at the bottom left hand corner of the game. They normally come in a disbtn for too, to allow players to know when they can not build that unit, or use that skill.
Portraits are part of a model that is shown left of all its stats. This is normally animated to show the unit talking and is mch more detailed sometimes.
So what do you mean?