Flash CS4 Professional.
bitmapItem.exportToFile(fileURI, quality)
fileURI A string, expressed as a file:/// URI, that specifies the path and name of the exported file.
quality A number, from 1-100, that determines the quality of the exported image file. A higher number indicates higher quality. The default is 80. New in Flash CS6 Professional.
A Boolean value of true if the file was exported successfully; false otherwise.
Method; exports the specified item to a PNG or JPG file.
Assuming the first item in the Library is a bitmap item, the following code exports it as a JPG file:
var imageFileURL = "file:///C|/exportTest/out.jpg";
var libItem = fl.getDocumentDOM().library.items[0];
libItem.exportToFile(imageFileURL);