External Image Preloader Help?

Hey,

Can anyone tell me how to attach a preloader to a mx.control.Loader, so when Click a thumbnail for an image a preloader can be called and displayed and load the external image??? Help Please!

Here is the AS on my buttons if it helps

[COLOR=SeaGreen]on (release){
unloadMovie(imageHolder);
loadMovie(“images/thumbnails/FullSize/12.png”,imageHolder);
}[/COLOR]