How do I turn the use of external images off for wiki icons?

Enter the URL of an image on the internet:

How do I turn the use of external images off?
Can I turn off this option?

Settings > VaultWiki: Aesthetics > Allow Custom Icons for Wiki Content? Uncheck "External Image URLs"

Thanks! Do I understand correctly that 'external' here means outside the wiki and not external domains?
I would like to allow images from my site.

It refers to the URL field in its entirety. I suppose it would be useful to have separate options for URLs to the same site vs URLs to other sites; however, it may be difficult to make a distinction because both XenForo and VaultWiki provide scripts to proxy external images locally. In this way, the user could enter a URL to the proxy script to bypass your "only images from the same domain" policy. Should this loophole be permitted, or should the proxy script be blocked completely as an acceptable icon source?

As long as the url entered is within the Board domain it will be good.

I think you missed my point, or you did not address it clearly: What if the user provides the following URL?

Code:

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http://wiki-domain.com/proxy.php?r=http://external-domain.com/actual-image.jpg

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In this case, the URL appears to be within the board domain, but the user is actually requesting an image from another domain. It would be difficult to catch this situation without following the URL, or without blacklisting known proxy scripts.

I think this needs to be disallowed, because the cache may not be permanent and there may well be copyright issues.