Perhaps I am wrong, but I think you will need to program an Extension.

The following Syntax will NOT work:

[http://ix.de [[File:2009-06-04_pic.png]]]

I have take more than 20 minutes to search http://www.mediawiki.org/wiki/Manual:Configuration_settings but I could not find anything to - use external images - create external hyperlinks with thumbnails

Therefore you have to write your own extension, which should not be too hard in this matter.

http://www.mediawiki.org/wiki/Manual:Extensions

Regards, Claus

PS: If you have never written an Extension, here is a short example:

--- $wgExtensionFunctions[] = 'example';

function example() { global $wgParser; $wgParser->setHook('example', 'process_input'); }

function process_input($input, $args, $parser) { return 'Hello World! Your input: ' . htmlspecialchars($input); } ---