If you could get away from the full color pictures you could go with screen printing; which you can get started for a very good price. Otherwise you probably would need to go with digital heat transfer (again a very good price to get started), but heat transfer is the least robust method to print with and leaves a very heavy hand on a dark color garment.
You also might check into having your shirts contract printed by a company that has a white ink DTG.
You also might check into having your shirts contract printed by a company that has a white ink DTG.