I have the issue somewhere, I can dig around and find it. They usually listed the cover's artist in the front page of the magazine, so it could possibly tell us who drew that artwork, too. The Sonic art on the birthday card is also in the official Sonic 1 and Sonic 2 strategy guide by Sega.
Buyatari's Sonic Art Pics | Page 3 | Sonic and Sega Retro Forums
I think he asked because it appears very similar to existing Sonic CD art in style. This isn't bad for a hack, but it could pose some potential confusion in this particular game. I was experiencing the same confusing when I played through the first level of Sonic Nexus, but I received various assurances that the graphics were, indeed, not nicked from Palmtree Panic. Fair enough.
And that isometric concept art just screams Sonic RPG to me. I'm surprised we haven't got any Jade Gully tunes yet, seeing as it was the most featured zone in magazine previews. But great find (s), and hope Senn has even more light to shed on the matter of Sonic Xtreme.
The benefit of all this was that Sonic being able to laugh at itself between this, the Boom cartoon and some of Pontaff's better jokes, showed that whether the games were great or terrible, a lot of the people making them were just along for the ride.
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IGN: Sonic Team Head: Pixel Art Won't be Viable Art Style for Sonic Games in 10 - 20 Years