code: private static void checkProperty() {
  		if (!pngLoaderPropertyChecked) {
  			pngLoaderPropertyChecked = true;
  
  			try {
  				AccessController.doPrivileged(new PrivilegedAction() {
  		            public Object run() {
  						String val = System.getProperty(PNG_LOADER);
  						if ("false".equalsIgnoreCase(val)) {
  							usePngLoader = false;
  						}
  						
  						Log.info("Use Java PNG Loader = " + usePngLoader);
  						return null;
  		            }
  				});
  			} catch (Throwable e) {
  				// ignore, security failure - probably an applet
  			}
  		}
  	}
docs: Check PNG loader property. If set the native PNG loader will
  not be used.