Control background image with JS - Active Desktop ?

Hi to all,
I'm trying to have a script change the background image of an
activedesktop page. Works fine when the page is displayed in a regular
navigator window (IE6SP1), but I get a "document.body is null or is not
a object" when the page is displayed as a wallpaper.

My line :
document.body.background = ImageList[counter];

Any idea ?

I have not been able to "attack" it through CSS. Is it possible ?

Thanks a lot in advance !
Damien