I do know a s:BitmapImage can load files on the fly with its source property [1]. It supports "The name of a external image file" and " The image location can be specified via a URL, URLRequest, or file reference". It is limited to PNG, GIF, or JPG.
[1] http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/spark/primitives/BitmapImage.html#source -Mark
