First, create an image that fits the allocated space. As of WordPress 2.8, the logo is a 30 pixels wide and 31 pixels tall transparent GIF. When using a transparent GIF or 8-bit PNG, ensure that the image matte matches the header background color: hex value 464646.

A logo named “custom_logo.gif” inside the template directory’s image subfolder can substitute the default WordPress logo with the following code inside the theme’s “functions.php” file.

Thanks to Smashing Magazine.

//hook the administrative header output
add_action('admin_head', 'my_custom_logo');
 
function my_custom_logo() {
   echo '
      <style type="text/css">
         #header-logo { background-image: url('.get_bloginfo('template_directory').'/images/custom-logo.gif) !important; }
      </style>
   ';
}