Sonic The Comic (Fleetway) often gave away posters or included them with the comic.
The first one is SuperSonic, but keep in mind that in their continuity, he was evil because Chaos Emeralds were originally scripted as evil. So naturally collecting them all would turn you evil too. Who knows what they did when ST finally hashed out the real story.
The next is a "Star Pin-up" featuring Tails. He is having a dream, but you can tell what the artist REALLY wanted to draw: the monster lizard. It is far more detailed and shaded than anything in the Sonic world, and so seems out of place.
The last is "Poster Mag #3" which features Sonic kicking through some familiar badniks like Caterkiller, Batbrain and Aquis. The last is a set of 3 mini posters based on Sonic Adventure 1 stock arts.