Resize external image?
Fragment of a discussion from Project:Support desk
To use images from Wikipedia on your wiki, you'll first have to configure $wgForeignFileRepos. Then you can use wikitext to embed them in pages and change their size as Varnent points out.
If the file(s) you'd like to display from Wikipedia were uploaded to Wikimedia Commons, you can use this setting to do the file repo configuration for you: $wgUseInstantCommons
