http://labs.bigspaceship.com/blog/?p=8 

Useful way to cache images in flash via the BitmapData class.

The idea behind the ImageCache is to load an image one time and then never again. Once loaded, store its BitmapData in an invisible movieclip, where it can be retrieved forever (memory permitting, of course). The BitmapData for said image is availible and accessible via its file path.