I want to change background image of sky providing custom image but it is not changing

var scene = viewer.scene;
scene.skyBox = new Cesium.SkyBox({
sources : {
positiveX : 'images/galaxy.jpg',
negativeX : 'images/galaxy.jpg',
positiveY : 'images/galaxy.jpg',
negativeY : 'images/galaxy.jpg',
positiveZ : 'images/galaxy.jpg',
negativeZ : 'images/galaxy.jpg',
}
});

In local system i have given these path but in sandcastle demo i have not added image.

Sandcastle Link

Any suggestions, Thanks