Change the Logo Monobook Skin - Mediawiki
The logo that appears in the top left of each page is determined by the $wgLogo configuration setting in the LocalSettings.php file.
Note: Most site administrators at Evergreen will not have access to edit this file. Please contact your System Administrator when you're ready with your new logo.
To change this you simply need to change the value of $wgLogo to point to the URL of your own logo image. You can upload a file via the wiki and use that address (which allows it to be replaced easily, so you may want to protect the page if you use this method) or use an image uploaded to your server via other means.
Note: It is possible to simply overwrite the default logo installed with MediaWiki, but this is strongly advised against, as an upgrade may end up overwriting it or change the default location of this file.
Tip: The logo image should be 135 pixels square.
Source: mediawiki FAQ