code: public ImageDescriptor getImageDescriptor(String id) {
          ImageDescriptor imageDescriptor = imageDescriptors.get(id);
          if (imageDescriptor == null) {
              String pluginId = getDefault().getBundle().getSymbolicName();
              imageDescriptor = AbstractUIPlugin.imageDescriptorFromPlugin(pluginId, ICON_PATH + id);
              imageDescriptors.put(id, imageDescriptor);
          }
          return imageDescriptor;
      }
docs: Call this method to retrieve the (cache) ImageDescriptor for the given
  id.
  
  @param id
  the id of the image descriptor or relative icon path if icon
  is inside of default icons folder
  @return the ImageDescriptor instance.