The following is just my interpretation of the cover art, but just to be safe I'll put it under spoiler tags.
In the picture, Harry and Voldemort are probably reaching for something (Harry perhaps saying "accio" something and LV trying to stop him from getting it) during their presumably final battle. That's the most likely suggestion of what this picture indicates.
But MY very first reaction was, "does this mean that there is something above even Voldemort in terms of evil?" With the colosseum like motif, it looks like Gladiator, with Harry and LV being watched by a higher power than either of them. And in a weird way, it's almost like Harry and Voldemort might be working together.
I'm almost surely wrong about there being a bigger evil, but I'm just glad we have something new to speculate on.
And now July 21'st once again can't come fast enough for me.
My guess was that it isn't Ollie. During the rather entertaining fight at the wedding, someone switched places with Ollie using the teleporting guy. That's gotta be the reason they made a big deal outta him.