I have to imagine this is the same root issue as noted here with the js clusterer library: <a href="https:&#x2F;&#x2F;github.com&#x2F;googlemaps&#x2F;js-marker-clusterer&#x2F;issues&#x2F;59" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;googlemaps&#x2F;js-marker-clusterer&#x2F;issues&#x2F;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&#x27;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.