The images below are the default images used in the marker clusters. The blue image on the left is used when there are only a few locations, the purple image on the right is used when there are thousands of locations, and everything in between is used when there are a few dozen, or hundreds of locations to merge together.
You can see an example of this here.
If you want to modify them, then you can download a zip file with the cluster images here. It’s important that the dimensions and the file names remain the same.
Set a Custom Image Path
If it doesn’t already exist, then first create a
wpsl-markers folder in your active theme folder. Move your custom images there, and place the code below in the functions.php.
This changes the default image path from
https://cdn.rawgit.com/googlemaps/js-marker-clusterer/gh-pages/images/m to the wpsl-markers folder holding your custom images on your own server.
After you made the changes the custom cluster marker images should show up on the map.