Manual:$wgLogo

Details
The URL of the site logo. Either a fully-qualified URL or a relative path from DOCUMENT_ROOT. The logo is expected to be 135 x 135 pixels, but this can be increased by modifying the site CSS (and smaller sizes are possible with no changes). It is generally a good idea to place this at the end of LocalSettings.php (before the ?> however), especially if you are using a variable such as $wgStylePath or $wgUploadPath.

It is possible to simply overwrite the default logo installed with MediaWiki, but this is strongly advised against, as an upgrade may end up overwriting it or change the default location of this file.