Ah, not modern skin not has logo, so you can add that line to MediaWiki:Monobook.css
2013/2/19 devunt devunt@gmail.com:
You can add this code in each skin's css file (ex: MediaWiki:Vector.css, MediaWiki:Modern.css)
#p-logo a { background-image: url('http://www.example.com/w/logo_you_want.png') !important; }