Hi Javier,
The current version of zagetti does not support WP3.5.1+ just yet. It now goes up to 3.4.2.
The issue’s you’ve got with the back-end functionalities like the media libraries, screen option and text editor are extremely likely to be caused by this incompatibility.
Our development team is currently working on an update for zagetti that will be 3.5.2 compatible. I don’t appear to have any issues with the menu as I check our demo but this could possibly also have something to do with the theme vs WP compatibility, or any existing issues might’ve been solved for the update
The area underneath the menu is a widget area, It’s called menu widget. You can add a text widget using a HTML image tag to display an image.
Hope this helps clarify some things
Kevin