As mentionned in comments, this is the updated solution in favor of Icon object with documentation.
Use Icon object
var icon = {
url: "../res/sit_marron.png", // url
scaledSize: new google.maps.Size(50, 50), // scaled size
origin: new google.maps.Point(0,0), // origin
anchor: new google.maps.Point(0, 0) // anchor
};
posicion = new google.maps.LatLng(latitud,longitud)
marker = new google.maps.Marker({
position: posicion,
map: map,
icon: icon
});