5 Stamps are not collected in levels and appear to be portraits of the 5 main characters (including Rosalina).These stamps are only unlocked when you beat all levels with each character. For instance, if you beat every level with Mario, you will get the Mario Stamp. If you play the game with multiple players, they all count for this challenge so long as they touch the end flag. This means that the shortest way to get all 85 Stamps is to play the game once with three characters playing at once, then play every level again with Rosalina and the remaining character.
