Change Google map marker

it is really simple - see http://code.google.com/intl/en-US/apis/maps/documentation/javascript/overlays.html#SimpleIcons

example straight from the google docs

var image = 'beachflag.png';
var myLatLng = new google.maps.LatLng(-33.890542, 151.274856);
var beachMarker = new google.maps.Marker({
  position: myLatLng,
  map: map,
  icon: image
});

if you want to do more complex stuff, you can also extend googles overlay classes and perform custom rendering ...