Former TheGIA staff member was kind enough to help confirm that the newer fanart might have been the first publicly posted, however.
Another candidate for the oldest Krystal fanart has been found. Can't confirm yet, but there's a pretty good chance.
A Nintendo Power envelope art from about two months after Dinosaur Planet was first revealed. Where are you, Leah from Las Vegas, Nevada?
DeviantArt is the closest thing we have to an image archive from that time. I found the oldest Krystal-related image, and the oldest Krystal fanart on DeviantArt.