Mainly because part I on death metal art was a zombie. Check few posts below. This goes with the same theme, though it is way more fantasy stuff. Don't take it too literally :D
EDIT: Zombies, ghouls and the living dead in general have quite a lot to do with death metal, now to think of it... Take Cannibal Corpse; The Undead Will Feast, Post Mortal Ejaculation, Unite the Dead, Mutation of the Cadaver and Pit of Zombies in example. Not to mention all those lyrical bases that point towards zombies and such from this band as well as many other DM bands out there. :)
But still, I quess this picture itself isn't brutal enough to be called "dm art". | 
