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). 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.