Manual:$wgFavicon

Details
The URL of the site favicon (the mini-icon which gets displayed in the browser's URL field and next to saved bookmarks). Uploading file directly to root folder of your domain/subdomain is sufficient (make sure file name is in lower case) else change LocalSettings.php and add following line where the path is either a fully-qualified URL or a relative path from DOCUMENT_ROOT. Example:

$wgFavicon = "$wgScriptPath/favicon.ico";

Tip: The favicon image should be either 16 x 16 pixels or 32 x 32 pixels.