Stephen Jones has put together a thoughtful posting on the Stuttgart Psalter image of Jesus being flogged: A 9th century depiction of Christ being scourged naked from behind with the scourgers’ fingers in the shape of the reversed 3 bloodstain on the Shroud!
Do read it. It is nicely done. I should mention, though, I don’t find the configuration of the fingers on a hand of each man doing the scourging very compelling. The whip mark distribution and the fact that Jesus is naked in the picture in a pose that seems shroud inspired is, to my way of thinking, more important.