Styling MarkerClusterer Icons?

What you need to do is use the url to specify which images to use instead of the blue/yellow/red images currently being used. And probably a good idea to include the height and width options too.

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
  }
];