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I used FlowPane to get side-by-side images. StackPane stacked the images on top of each other.

Did you try that?

Yes, but where does it position them?

What is the WxH of the gull.jpg image? Need that to consider the setViewport calls.

Why are you using the StackPane class? What does the API doc say about what that class does?

What is the...

Please post any code here wrapped in code tags that you want to talk about.

Also posted at: JavaFX Viewport Issues

Try removing the unused code and also some of the minor methods. Pass two objects to the StackPane constructor.
I did and was able to get the two images to...