A couple of days ago, I posted about another potential candidate for the oldest Krystal fanart. The publication date of the fanart was ambiguous, so I reached out to staff of TheGIA. Andrew Vestal was kind enough to do a little digging and according to him, the image would have been posted on October 4th, 2000. Fanarts were usually posted within a few days of receipt, so this fanart was probably not shared with them before October 1st, 2000.
So that means our previous champion retains her title by a wide margin! Leah from Nevada's Krystal fanart on the back of an envelope is the oldest known Krystal fanart. However, since that image wasn't published until the November 2000 issue of Nintendo Power, the newer image by Red Leader is the oldest publicly published Krystal fanart currently known.
As always, the search continues about this and all other interesting Krystal-related topics. So good to get a concrete answer about something found on the internet 20+ years ago!