You can use the MapIconMaker to create icons for your map markers easily:
<script src="mapiconmaker.js" type="text/javascript"></script>
MapIconMaker.createMarkerIcon and add it to the map.
var map = new GMap2(document.getElementById("map"));
map.setCenter(new GLatLng(37.441944, -122.141944), 13);
var newIcon = MapIconMaker.createMarkerIcon({width: 64, height: 64, primaryColor: "#00ff00"});
var marker = new GMarker(map.getCenter(), {icon: newIcon});
map.addOverlay(marker);
View example (createmarkericon-simple.html).
We've created a wizard to make it easy for you to play around with the various parameters in MarkerIconOptions. The wizard lets you choose the three colors and the size, and shows you the result using randomly placed markers on a map.