In JavaFx you can set the fitWidth of an image view programmatically to resize the image shown to match the resolution and size of the current user's monitor. However, you cannot express the desired fitWidth with CSS. Some people are using background images instead of imageView as a work around but this is not always convenient.

I suggest introducing two CSS properties fit-width and fit-height allowing me to write the following CSS for a button which uses an ImageView as its graphic.

.myButton .imageView {
   -fx-fit-width: 30;
   -fx-fit-height: 30;
}