Thanks to MapifyPro customer "Tom" for provided this brief how-to:
To customize the cluster icons, you need to edit this line in the ‘markerclusterer.js' file of the MapifyPro plugin:
MarkerClusterer.IMAGE_PATH = "http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclustererplus/images/m”;
The image path should be edited to point to your own cluster icon images. You need five different sizes of icon, the specifications of which can be taken from the images in the root folder on the Google server:
When creating your own icons, you need to append the relevant number. (The Google icons are m1, m2, m3 etc. and your custom icons might be cluster1, cluster2, cluster3 etc.).
Further Reading: http://stackoverflow.com/questions/4416089/google-maps-api-v3-custom-cluster-icon