Flash CS4 Professional.
fill.bitmapPath
Property; a string that specifies the path and name of the bitmap fill in the Library. This property is available only if the value of the fill.style property is "bitmap".
The following example sets the fill style of the specified item to a bitmap image in the Library:
var fill = fl.getDocumentDOM().getCustomFill();
fill.style = "bitmap";
fill.bitmapPath = "myBitmap.jpg";
fl.getDocumentDOM().setCustomFill(fill);