My guess as to why konohamaru knew what sasuke looked like is that he should have seen him in the village at least once before the time jump. everyone should know who the last uchiha member is in the village. so konohamaru just added some "age" to him to estimate his appearance, kinda like how they use programs to see how wanted criminals would look like when they are older to identify them.
But konohamarus "estimation" is dead on so im guessing kishi had to draw the real sasuke cuz then no one would know who it was if he drew someone who kinda looked like sasuke but not quite look like him.