change image cluster marker code example

Example: image clustering custom image

var clusterStyles = [
  {
    textColor: 'white',
    url: 'path/to/smallclusterimage.png',
    height: 50,
    width: 50
  },
 {
    textColor: 'white',
    url: 'path/to/mediumclusterimage.png',
    height: 50,
    width: 50
  },
 {
    textColor: 'white',
    url: 'path/to/largeclusterimage.png',
    height: 50,
    width: 50
  }
];