I'd love to see Running Wild, but agree that this image looks too dark for them. Besides, they're the wrong type of skulls (now there's something I never thought I'd say!), and there are no crossbones. White Skull? Again, I'd love to see them, but I'm not convinced they're big enough to headline Bloodstock. Viking theme? Well, Einherjer put on a decent show at The Underworld the other day, but they weren't able to fill it, so there's no way they could headline Bloodstock (although giving them the top slot on the second stage would be an excellent move, hint hint!).
So what else? Well I'm leaning towards Grave Diggerr, particularly given the Ring Of The Nibelungen theme to their last album, which ties in nicely with the Orcs and general Tolkeinesque imagery. Not to mention the huge gold ring in the middle of the image! Personally, I'd love to see them at Bloodstock. They haven't really put a foot wrong with their last 4 or 5 studio albums, and they completely stole the show when they supported U.D.O. at The Underworld a few years ago. They're a great live band.