var lib = fl.getDocumentDOM().library;
var libItemLength = lib[0].length;
for (i=0; i < libItemLength; i++){
     if(lib.getItemType(lib[0][i].name) == "bitmap") {
     	lib.selectItem(lib[0][i].name);
	lib.setItemProperty('allowSmoothing', true);
	lib.setItemProperty('compressionType', 'photo');
     }
}

download the jsfl file, and please share your comments and improvements.