Properly setting of externalGraphic size in OpenLayers.

gistfile1.js

var myStyles = new OpenLayers.StyleMap(
  new OpenLayers.Style(
    {
      externalGraphic: baseUrl +'/img/marker-blue.png',
      fill: false, 
      stroke: false, 
      pointRadius: 0,
      graphicWidth: 18 , 
      graphicHeight: 30,
      fillOpacity: 1,
      graphicXOffset: -30/2,
      graphicYOffset: -18/2
    })
  );