looks like a simple permission problem.

I know but the permission is 777 for the public directory!!
I don't understand the problem :(

Hi marcob2evolution,

There is a script to be found on [url=http://manual.b2evolution.net/Debugging#Permissions]this page[/url]. Copy and paste it in a new file that you call permission.php. Upload it to the root of your blog and call it in a browser (e.g. 'http://example.com/blogs/permission.php' if your media folder is 'http://www.example.com/blogs/media/'). The script wil output information about the settings of the media folder. Please report that information in this topic.
In the meantime you can read [url=http://manual.b2evolution.net/Directory_and_file_permissions]this interesting page[/url] with more general information about file permissions and the media folder.

Good luck