I have to imagine this is the same root issue as noted here with the js clusterer library: <a href="https://github.com/googlemaps/js-marker-clusterer/issues/59" rel="nofollow">https://github.com/googlemaps/js-marker-clusterer/issues/59</a><p>Essentially, the library was previously hosted on googlecode.com and they moved it to Github. However, the library had hard-coded links to resolvable urls rather than local image assets, thus meaning the cluster images didn't show up. Simply changing the URL to point at the assets on github or to a local copy of the images fixes up the issue.