tallis...For most sites it will just be "/" without the quotes. I really depends on how your webserver/host is set up. The easiest way is to login to your site using an ftp client and make note of the location that the ftp client puts you in, this is what would go into the base path. Another example would be "/public_html". If you refer to the manual regarding the "Media Directories" it will hopefully help you a little more.
Note..In a new version of Blogg-X this won't be an issue since I am working on a way to allow image uploads without using the ftp functionality.