Look at the attached image.

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.image.ImageView;
import javafx.stage.Stage;

public class Test extends Application {

    public static void main(String[] args) {
        launch(args);
    }
      
    @Override
    public void start(Stage primaryStage) {
        ImageView view = new ImageView("animdog.gif");
        Group g = new Group(view);
        primaryStage.setScene(new Scene(g));
        primaryStage.show();
    }
}

I looked on this in explorer, gimp, firefox: all show dog on white (actually with opacite background) background. JavaFX shows only first frame with opacite background and all other - with black background...