InstantCommons is a feature of MediaWiki to allow the usage of any uploaded media file from the Wikimedia Commons in any MediaWiki installation world-wide. InstantCommons-enabled wikis, like this one, cache Commons content so that it is only downloaded once, and subsequent pageviews load the locally existing copy.
With InstantCommons enabled, you can choose any image from Wikimedia Commons, click the "Use this file" button (the button with the wiki icon next to) and paste the markup into the page on this wiki you wish the picture to appear. An example, pasting
[[File:Wikipedia-Marker-commons.svg|thumb|A picture from Wikimedia Commons embedded in this Wiki]]
will render the thumbnail (as can be seen on the right side of this page).