XF 1.2 Background-image alignment issue

[Frode789]

I've added the following to my background-image:

no-repeat center fixed
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;

Works great in Wordpress, the image is completely covering the screen etc. However, in Xenforo the image "skips" up and down once every time I load the index page. I've tested in Chrome, IE and Firefox. Here is the link for you to check:
http://www.worldofdota2.com/forums/

Any ideas why?