Flash MX 2004.
bitmapItem.compressionType
Property; a string that determines the type of image compression applied to the bitmap. Acceptable values are "photo" or "lossless". If the value of bitmapItem.useImportedJPEGQuality is false, "photo" corresponds to JPEG with a quality from 0 to 100; if bitmapItem.useImportedJPEGQuality is true, "photo" corresponds to JPEG using the default document quality value. The value "lossless" corresponds to GIF or PNG format (see bitmapItem.useImportedJPEGQuality).
The following code sets the compressionType property of the first item in the library of the current document to
"photo":
fl.getDocumentDOM().library.items[0].compressionType = "photo";
alert(fl.getDocumentDOM().library.items[0].compressionType);