How do I change default MediaWiki logo

From PaulGuWiki

The logo that appears in the top left of each page is determined by the $wgLogo configuration line in the LocalSettings.php file.

There are two ways to change the logo:

Upload a picture with your wiki and use that address. This allows the photo to be replaced easily, so you may want to protect the page if you use this method.

Then add the $wgLogo line to LocalSettings.php, for example:

$wgLogo = 'http://yourwiki.org/wiki/images/6/62/photoname.jpg'; 

Or upload an image to your server by other means (such as FTP). Add the $wgLogo line to LocalSettings.php, for example:

$wgLogo = "{$wgScriptPath}/photoname.jpg"; 

In this example, the photo is in the same folder as the LocalSettings.php file.

It is generally a good idea to place this at the end of LocalSettings.php, especially if you are using a variable such as $wgStylePath or $wgUploadPath.

Caution: Do not simply overwrite the default logo installed with MediaWiki (/skins/common/images/wiki.png); this file will be overwritten when you upgrade.