I need your "magic" help, and it's kinda urgent

[COLOR=slategray]I’m head of design for a norwegian youth festival called “Fest 07”. One of the city’s churches is doing a really nice thing for the the neighbourhood: cleaning up streets, painting houses for free, free appartment cleaning and stuff like that. Also there’s concerts, happenings etc…[/COLOR]
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[COLOR=slategray]I need some help with something for the festival website, and I’m kinda short on time. (This is no payed job, I also work for free.)[/COLOR]

This is what I’m gonna make:

The site background is an image (it fills the browser). Every time you click a navigation link a new image loads behind it, and the image on top is masked out (a tweened mask) when the new image has finished loading.

I need to be able to define the url to the new image when clicking a nav link.

[COLOR=slategray]Could anyone point me in the right direction (I understand the basics of AS), or maybe even help me create this? The site needs to be online monday.[/COLOR]