Do you want high res Sonic Adventure art?
Duh, of course you do.
The recently-started Video Game History Foundation has begun archiving media assets sent to press from game companies. Among the first uploads is a zip disk of "Sonic Adventure character art", which includes 3 super high resolution pieces of promo art from the game. Check 'em out:
(Click the pics for full quality.)
Look at that. You can even count the polygons in Sonic's shoes. Amazing, right?
A copy of the disk was uploaded to the Internet Archive. The ZIP file includes an ISO image of the original disk, plus the art files converted to PNG.