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 135px square, 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.