Bug 195335 Multithreads issue
a static hashmap in class HTMLServerImageHandler has not been synchronized which may leads to that the same images are handled(eg: write to disk) multiple times.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/HTMLServerImageHandler.java
* @see org.eclipse.birt.report.engine.api2.IHTMLImageHandler#onCustomImage(org.eclipse.birt.report.engine.api2.IImage,
	 *      java.lang.Object)
	 */
	public String onCustomImage( IImage image, Object context )
	{
		return handleImage( image, context, "custom", false ); 
	}
	/**
	 * returns a unique file name based on a directory and name prefix
	 * 
	 * @param imageDir
	 *            directory to store the image
	 * @param prefix
	 *            prefix for the file name
	 * @return a file name
	 */
	protected String createUniqueFileName( String imageDir, String prefix )
	{
		return createUniqueFileName( imageDir, prefix, null );
	}
	
	/**
	 * creates a unique tempoary file to store an image
	 * 
	 * @param imageDir
	 *            directory to put image into
	 * @param prefix
	 *            file name prefix
	 * @param postfix
	 *            file name postfix