KevinL is correct on this matter, besides Blizzard/WoW is pretty friendly on allowing people to use images from there websites and in fact have made fansite kits for people to help them build there fansites. Inside those fansite kits are a lot of images from the official website it self so the theme you are talking about could have been using a fansite kit as well.
The only time they get upset is if you are making an exact duplicate of there site and trying to steal people's account info by pretending to be the official site and having them login with there real account info. This is mostly done in account harvester email campaigns from someone using a spoofed email header pretending to be an official Blizzard email and linking to a harvester duplicate site.
So you should be in the clear as long as you are not doing this.