In Flash, the BitmapData object can be used to store bitmaps in RAM, you can later draw them to a MovieClip using the beginBitmapFill() method.
How do you load an ex
You have to load the external file (.jpg) into a MovieClip and wait for it to load.
Then take a snapshot of the MovieClip that contains the external file, into your BitmapData object.
myBitmap = new BitmapData(myMC._width, myMC._height,true,0x00FFFFFF)
myBitmap.draw( myMC)
See Introducing the Image API in Flash 8.