iOS SLComposeViewController : I can not post an image as PNG format

Question

When I post a png image to Twitter by using SLComposeViewController, the posted image is converted to JPEG image automatically. As a result of that, the image quality is degraded. Do you know how to post a PNG format.

Any help will be gratefully appreciated.

- (void)postTwitter
{
    SLComposeViewController *controllerSLC = [SLComposeViewController composeViewControllerForServiceType:SLServiceTypeTwitter];
    [controllerSLC setInitialText:@"Test post from my iPhone app"];
    [controllerSLC addURL:[NSURL URLWithString:@"http://www.apple.com"]];
    [controllerSLC addImage:[UIImage imageNamed:@"test.png"]];
    [self presentViewController:controllerSLC animated:YES completion:Nil];
}