As Henkka says in this interview (at c. 4:40), he gets involved with "the imagery and aesthetics (of the band) ... the merchandise and artwork", so although he probably doesn't do the actual designs (or maybe he does!), he's clearly closely involved
[ame]http://www.youtube.com/watch?v=mLhSz-0hNKM&feature=player_embedded[/ame]