Logo doesn't appear on the Seven theme on a fresh installation
The logo for the theme selected is added to a php serialized array in the system table under the info column. It is advisable to not use FTP to upload the image but instead of the Admin UI under the themes section to do so.

Click Appearence > Seven 7.8 > Settings

Then scroll to "Logo image settings" and uncheck "Use the default logo".

Click "Choose File" and pick the logo image file.

Click "Save configuration" and your logo should show up in the site. If caching is enabled, the caches might have to be cleared. 

Edit: The page.tpl.php file for the seven theme does not have a section to display the logo. Not sure if this is a bug or just a design decision. Either way, there is no way to show a logo in the seven theme at this time.