Обратимся к документации: Image.getGraphics: Creates a new Graphics object that renders to this image. This image must be mutable; it is illegal to call this method on an immutable image. The mutability of an image may be tested with the isMutable() method. Image.createRGBImage: Creates an immutable image from a sequence of ARGB values, specified as 0xAARRGGBB. Если б всё так было просто, в PPM сразу бы работу с прозрачностью сделал.