If the earth needs a flag, then the flag has the purpose to identify the earth and indicate the origin of an earthling. Thus, it should look like the earth as s

If the earth needs a flag, then the flag has the purpose to identify the earth and indicate the origin of an earthling. Thus, it should look like the earth as seen from space. Which would result in the borders of our continents. This would be boring for us, but it would make sense for aliens.