Manual:$wgEnableUploads

Details
When set to true, MediaWiki will allow users with the 'upload' right (logged-in users by default) to upload images and other enabled files. To enable image uploads, make the images directory writable by the web server, then set  in LocalSettings.php.

IMPORTANT: Uploads need to be set up properly in order to be secure!

Note: Prior to v1.5.0, $wgDisableUploads was used instead.