I believe that this will not work. Processing concatonates all .pde files into a single text file and then transpiles that code it into a single Java class (PApplet) – so your ImagenSerializable will end up becomming an inner class. You cannot put it in a .pde tab. You need to separate your ImagenSerializable class from your processing sketch – not .pde file, but written in a separate .java file.