Finally I found where was the problem. In fact Phalcon was doing it right all the time and the problem was not about the code of the controller or the model but with the db table.
The table:
CREATE TABLE news (
news_id INT(10) NOT NULL AUTO_INCREMENT,
title VARCHAR(255) NOT NULL DEFAULT '',
body TEXT NOT NULL,
image VARCHAR(255) NULL DEFAULT NULL,
)
The problem: I wanted the "body" field not to be required and used skipAttributes() in the model but it wasn't working. On the other side I didn't get any errors about the requirement of the "image" field about which I didn't specified anything in the skipAttributes().
The solution: I just had to set "body" as DEFAULT NULL - that's all :)