Manual:$wgEnableImageWhitelist

Usage
If is false, enabling this will turn on an on-wiki external image whitelist at MediaWiki:External image whitelist (a page on the wiki). Items on this whitelist page are regular expression fragments that are each matched when an external image is being parsed. Items that match a whitelist entry will be displayed, those that do not match will be linked.

Lines on the whitelist starting with # and blank lines are not used to find a match

This setting may be used concurrently with without any issues.

The following code snippets show how to enable and disable this feature, respectively.

An example MediaWiki:External image whitelist page
^http://upload\.wikimedia\.org/ ^http://.*?\.photobucket\.com/
 * 1) This line is a comment. The part below allows all images from commons
 * 1) Allow Photobucket