Getting logo's url

just make the source of your site (CTRL+U firefox).
In that code you might see some code like this.

   <div id="header">
            <a href="/" title="Index page"><img id="logo" src="/themes/bluebeach/logos/drupal.org.png" alt="Logo" /></a>
    
    <div id="tabfloat">

copy the source of your logo.
Open a new tab then enter your site name just like drupal.org then paste the source of your logo in the browser and enter, you will get your logo.
eg: http://drupal.org//themes/bluebeach/logos/drupal.org.png
This is the logo path of drupal.org

Hope this will help.
Cheers.