import com.adobe.images.JPGEncoder;

var jpgSource:BitmapData = new BitmapData (sketch_mc.width, sketch_mc.height);
jpgSource.draw(sketch_mc);

var jpgEncoder:JPGEncoder = new JPGEncoder(85);
var jpgStream:ByteArray = jpgEncoder.encode(jpgSource);

var header:URLRequestHeader = new URLRequestHeader("Content-type", "application/octet-stream");
var jpgURLRequest:URLRequest = new URLRequest("jpg_encoder_download.php?name=sketch.jpg");
jpgURLRequest.requestHeaders.push(header);
jpgURLRequest.method = URLRequestMethod.POST;
jpgURLRequest.data = jpgStream;
navigateToURL(jpgURLRequest, "_blank");