Small JavaFX application for viewing png files.

PngImageViewer.java

import java.io.File;
import java.net.MalformedURLException;
import java.util.logging.*;
import javafx.application.Application;
import javafx.beans.value.*;
import javafx.event.*;
import javafx.geometry.Bounds;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.image.*;
import javafx.scene.layout.Priority;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.FileChooser;
import javafx.stage.Stage;
 
public class PngImageViewer extends Application {
  @Override public void start(Stage primaryStage) {
    final ImageView imageView = new ImageView();
    Button imageChooser = createImageChooserButton(imageView);
    ScrollPane scroll = makeScrollable(imageView);

    VBox layout = new VBox(10);
    layout.setStyle("-fx-background-color: cornsilk; -fx-padding: 10;");
    layout.getChildren().addAll(
      imageChooser, 
      scroll
    );
    VBox.setVgrow(scroll, Priority.ALWAYS);

    primaryStage.setScene(new Scene(layout, 800, 600));
    primaryStage.show();
  }

  private Image choosePng() {
    try {
      FileChooser fileChooser = new FileChooser();
      FileChooser.ExtensionFilter extFilter = new FileChooser.ExtensionFilter(
        "PNG files (*.png)", 
        "*.png"
      );
      fileChooser.getExtensionFilters().add(extFilter);
      
      File imageFile = fileChooser.showOpenDialog(null);
      if (imageFile == null) {
        return null;
      }
      
      String imageFileLoc = imageFile.toURI().toURL().toExternalForm();

      return new Image(imageFileLoc);
    } catch (MalformedURLException ex) {
      Logger.getLogger(PngImageViewer.class.getName()).log(Level.SEVERE, null, ex);
      
      return null;
    }
  }
  
  private ScrollPane makeScrollable(final ImageView imageView) {
    final ScrollPane scroll = new ScrollPane();
    final StackPane centeredImageView = new StackPane();
    centeredImageView.getChildren().add(imageView);
    scroll.viewportBoundsProperty().addListener(new ChangeListener<Bounds>() {
      @Override public void changed(ObservableValue<? extends Bounds> ov, Bounds oldBounds, Bounds bounds) {
        centeredImageView.setPrefSize(
          Math.max(imageView.prefWidth(bounds.getHeight()), bounds.getWidth()), 
          Math.max(imageView.prefHeight(bounds.getWidth()), bounds.getHeight())
        );
      }
    });
    scroll.setContent(centeredImageView);
    return scroll;
  }

  private Button createImageChooserButton(final ImageView imageView) {
    Button imageChooser = new Button("Choose Image");
    imageChooser.setOnAction(new EventHandler<ActionEvent>() {
      @Override public void handle(ActionEvent t) {
        Image chosenImage = choosePng(); 
        if (chosenImage != null) {
          imageView.setImage(chosenImage);
        }  
      }
    });
    return imageChooser;
  }
  
  public static void main(String[] args) { Application.launch(args); }
}

Answer to Oracle JavaFX forum question: FileChooser something is wrong with the path Input Stream must not be null