PageSnapshots is a MediaWiki extension.
Status for Mediawiki4Intranet distribution:
After installation, you'll see "| snapshot" link in each row of page history. Click it to view a snapshot of this page with corresponding old versions of images and templates. Note that the extension can't display images and templates that were renamed or removed, so avoid removing anything if you want to use snapshots in the future.
As usual, just add the extension in your LocalSettings.php: