Bug 190981 The image id of image in different pages are the same
The id of image generated by the same report item (such as chart) are the same, thought the image contents are in different pages.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/Image.java
*            the source stream
	 * @param tgt
	 *            the target stream
	 * @throws IOException
	 */
	protected void copyStream( InputStream src, OutputStream tgt )
			throws IOException
	{
		
		byte[] buffer = new byte[1024];
		int size = 0;
		do
		{
			size = src.read( buffer );
			if ( size > 0 )
			{
				tgt.write( buffer, 0, size );
			}
		} while ( size > 0 );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.engine.api.IImage#getExtension()
	 */
	public String getExtension( )
	{
		return extension;
	}
}