We had a supposed leaked image earlier today, and I can tell you its fake. Its not real. Here is a description of the real cover from someone that received their White Dwarf early.
While this is a still a rumor until something official is announced, I am giving it the full 5 stars of reliability.
via an anonymous source on Faeit 212 *****
The White Dwarf came early here. The cover is a large dwarf (runelord?) wearing a winged helmet, furred cape, and wielding a large two handed hammer. I dont know fantasy very well, but I am guessing it is a runelord. There is cracks of lightning surrounding the hammer's ornate head. Along one hip is a smaller hammer, and along the other a book with a large dwarven rune upon it.
Overall I like the cover, and is what I would expect for the cover. He is marching foward like he is going to whack you with his hammer, and it looks like it would hurt.