Permission to upload images for a role

Hello, 

I added a "staff" role to allow the content manager of the site inserting new content on a site driven by Yamato.

When the user "staff", which has "staff" role, click the "image" button on ckeditor the system correctly open the "image properties" window; but when he click the "browse server" button he gets an access denied page.

I granted to the role more permission than necessary to debug this, but no effect so far.

Se the attached screenshot, please help

Simone