Manual:$wgAllowCopyUploads

Details
Allow for upload to be copied from an URL. Requires.

Users also need to have the associated user right 'upload_by_url' before this option will show up on Special:Upload. By default only administrators (sysops) have this permission.

Usage
Add to the bottom of your LocalSettings.php: (The above example will let all autoconfirmed users upload by url).