You can easily enable CDN in Joomla by following these instructions:
1. Download the latest version of Joomla CDN extension.
2. In Joomla control panel, choose Extensions -> Install/Uninstall.
3. Download the extension through your browser.
4. Go to Extensions -> Plugin Manager.
5. Choose System – CDN plugin for Joomla!.
6. Set the extension as active and write the CDN URL in CDN URL Domain.
Check the Joomla source code that the site uses CDN. Picture links should be replaced with CDN URL address.