Obviously, the game needed to be hyped before release, and screenshots were released, but some of those are different from the final, and even the beta. =0
This image was taken from an advertisement on the back of a Sonic the Hedgehog 2 manual. Mickey's cloak is a solid colour, which is a dead giveaway, but what's interesting is that Mickey is spreading his cape while jumping, but is playing his standing one. Either they used Start to pause him and move him up, his standing + cloak animation was instead of his jumping + cloak one then, or that rock in the background was actually something you could stand on.
Also note: No health! =0
This is also from the same advertisement. The area graphics seem much, much more simplified, as there are barely any details in the background, and the ground's details are minor. It also appears that the ripple effect wasn't implemented yet, as the castle is perfectly straight there.
Also from the advertisement. No difference from the beta here.