Marble has support for the KML IconStyle tag:

https://developers.google.com/kml/documentation/kmlreference#iconstyle

- Using Google Earth create a placemark and assign a custom icon to it.

- Then save the placemark into a kml file and load it into Marble

- Compare the result in Google Earth and Marble

- Then - step by step - change the <scale> and <hotspot> properties in Google Earth and compare in Marble.

Behavior that differs in both applications should get checked against the KML Reference specification. In case there is a bug it should get reported.

Create a fix for the bug.