In the cofiguration I want to put the path for the uploading files but it accepts only the \ and not the / so when I'm writting http:/xxx.gr/httpdocs/joomla it's turning it in http:\xxx.gr\httpdocs\joomla and of course this path is not correct and gives me an error message that the path is not correct. Any idea what I have to do?

Hello,
the upload path must be a system path (directory path) not an URL!
it is a directory where the uploaded files are stored on the server.

(e.g. var/www/uploads)
For security reasons (so the files cannot be downloaded directly) this directory should be outside the Joomla root directory. Or protected by .htaccess or at server configuration level to prevent direct access in case when you cannot place it outsite the Joomla root. (e.g. on some shared hosting servers)