Those Sonic images were taken from other sources and are not original like the pictured confirmed Melee characters.
Even if this is true, that doesn't necessarily mean the ads are "fake". It just means that they were made by taking pictures and slapping them together. The articles are already reporting that these are mockups made by the store - not by Nintendo. So without access to original Nintendo art, they wouldn't have any other choice but to take pictures from other sources. Pre-orders are a huge deal.
So I'm still not convinced either way.
EDIT: Okay, I think this article best explains what is going on here. The ads are real, but they may not be accurate. They're just careless Gamestop and Best Buy mockups. It's not a confirmation, but it helps.
http://www.sonicstadium.org/sonicnews/315/