You'll have to maintain the same name states as the existing marker set. So replace the default markers with your own but keep the names like marker_00.png, marker_01.png, etc.
Since the size of your marker will likely be different than the default set you will also need to specify the marker attributes as well as create a custom shadow. Those attributes are in the map_assets/js_css/xml_g_maps.js script file.
This resource will calculate all those values for you:
http://www.cycloloco.com/shadowmaker/