Are these server and client programs defined using .pde sketches? Are you defining the ImagenSerializable class inside each sketch?
I’m just guessing, as I don’t have experience with this kind of serialization (and haven’t seen example code froom you) but I think you need to separate your ImagenSerializable class from your processing sketch – e.g. in a java file – and import it. Otherwise your class will be compiled by Processing as an inner class of your PApplet, which I believe (not sure) will result in you serializing the whole enclosing PApplet object unless the inner class is static. Then you would have two objects whose types don’t match – and which are unnecessarily large.