I am using .gif instead of .jpg as backgrounds in the sprites. It works fine and there's no problem there. The point is that these gifs are animations and they don't play in webkit browsers. They only play while the page is being scrolled.
A link to what I mean: http://www.meits.es/nadal2012
You can see the effect I'm talking about in the second slide
Hi, in every background image try to delete "fixed"