You are over thinking it Sarah. For a show like this it is best to just go with the main most important characters to the show overall. You also may not have promotional pictures for all the characters you are thinking of
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As for ordering not everyone on the team does their graphics the same way with a character in the middle and everyone on either side. It depends on the show, pictures, and team member. Personally I prefer to know which characters need to be next to each other and any who should not be next to each other and then go from there seeing what I find works best.