Hmmm... interesting. That's a bug. Thanks for reporting, Davide :)
I'm actually surprised you have an image name with à in it. Since WordPress normally transforms such characters during upload, I never thought of escaping the image location. A mistake on my part.
Could you go to http://wordpress.org/plugins/xml-sitemap-feed/developers/ and download the development version? Then uninstall the current plugin from your site and install the zip via the plugin installer uploader. Or unpack and overwrite via FTP ...
Please let me know if that fixes the error in GWT.