Question: I'm changing my background-image css property using Mootools:

**$(document.body).setStyle('background-image','url(' + pBackground + ')');**

*And it's its working, but how can a make one fade effect between picture change?*

Thanks,
Pedro

Answer: 
Just as a caution, any child elements of that div will also fade, so if you want the background to fade while elements over it remain opaque, you will need to absolutely position any child elements.

Absolutely positioning all elements brings other problems with it when you have variable length content, but there are ways around that too.
Not sure of what i am saying, BUT since it's not a part of the html document, it's not an 'element' so javascript should not be able to work on it.

But, just an idea, and depending on how your site looks, you could try to set an opacity, to simulate an opacity on the body, which can lead to the effect you want..