Thank you very much for your support!
I am using another plugin called “members list” that hasn’t got the problem with the “feature image” and, most of all, because in this way I can visualize the articles written by the site’s visitor in the home page and not only in the sidebar.
But your answer could be very useful because I’m trying to create a new custom post type in order to create a difference between my articles and that written by “the others”.
🙂