I know it says to not add a pic if there already is one
You can send replacement pics, but only if they're a significant improvement. If the picture you send is inferior or equal to the picture we already have, I won't change it.
For example a pic which is much clearer. Or an english poster when we only have a poster in foreign language. Or a more current picture of a person.
but can we add a character screenshot pic even when there's a celebrity actor pic already there?
Of course! The person picture and the character picture are distinct pictures. If there's no character picture, you won't replace anything.