The decals are a new feature of SC2 I really like. Basically my war3 map was using my logo on many places, shoulders, flags, buildings and i didnt even know SC2 had such things. So I can say I thought of decals myself but they are not white and black colors. So we will need to see a good way to edit an image and make it black-white to be a decal.
Im sorry I dont think you can change the decal ingame with triggers, just like you cant change player settings like change race, change status from Comp to say User, etc using triggers.