Asi jsi blbe hledal, v dokumentaci je tohle:

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class AA extends Application {

    @Override public void start(Stage stage) {
        Image image = new Image("buce.jpg");

        ImageView imageView = new ImageView();
        imageView.setImage(image);
        imageView.setFitHeight(300);
        imageView.setPreserveRatio(true);
        imageView.setSmooth(true);
        imageView.setCache(true);

        Group root = new Group();
        Scene scene = new Scene(root);
        scene.setFill(Color.BLACK);
        HBox box = new HBox();
        box.getChildren().add(imageView);
        root.getChildren().add(box);

        stage.setTitle("ImageView");
        stage.setWidth(800);
        stage.setHeight(600);
        stage.setScene(scene);
        stage.sizeToScene();
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}

...a to samozrejme funguje.