Guys, tcdbkrn solved this thing two days ago. The YouTube highlight video he posted clearly shows Mark Whiten with sleeves and gloves that match the baserunner on the card (check timestamp 3:01). Even the dirt on his right pant leg is a match. Other cards show Whiten with cleats matching the baserunner on the card.
for example: https://www.tcdb.com/ViewCard.cfm/sid/76927/cid/5517635/1991-Score-12th-National-Sports-Collectors-Convention-6-Mark-Whiten
Don't be confused by the Getty images of Glenallen Hill which, as tcdbkrn pointed out, are helpful in confirming that the game is indeed 5/11/91 (you can even match up some of the fans' outfit colors), and eliminating Hill as the runner on the card, as his helmet ear flap and (lack of) sleeves are conclusively different from the runner on the card.
According to baseball-reference, Merullo took seven pickoff throws with Whiten on first base that day (4 in the 2nd and 3 in the 8th). There were 10 pickoff throws with Hill on first (all in the 8th). I'd guess the same photographer (Rick Stewart) took all the photos, and probably all in the bottom of the 8th inning.
100% it's Whiten on the card.