I added four icons to a Stage before I show it with resolutions form 16x16 to 128x128. But the icon shown in the windows task bar is very blocky. The exact same icons set on a Swing JFrame result in a smooth icon in the windows task bar.

The docs say: "The images should be different sizes of the same image and the best size will be chosen", but it appears to be selecting the last icon in the list without regard for the size.