Manual:$wgAllowExternalImages

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Whether to allow inline images hosted on external websites, sometimes called inline linking.

Details
When set to, users can bring in an inline image from an external URL. They do this simply by sticking the full URL into the wiki text (without any other surrounding markup being required) That is the direct URL to an image resource on another website.

When set to, the full URL appears as a text link to the image.

Note that if you want to allow images from a single trusted source (e.g. from within your own domain) then set this to false, and use.

Example:
Using wikitext:

Result: https://en.wikipedia.org/upload/b/bc/Wiki.png