In order to change the background of a JavaFX Scene object you have to change a CSS file, which is programmatically associated with corresponding Scene.
In below example the background is set to an image, which is placed in the same directory as the CSS file. I do not think this advisable in a real project where you would have it in a bin directory, but for example purposes it is ok.
The CSS file is set in the Scene object as follows.
The result looks quite nice, but I guess that depends on the picture as well ;0).