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 Wiki 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.