You can easily setup CDN in drupal by following these instructions:
1. Download the latest version of Drupal CDN.
2. Unzip it to Drupal modules folder.
3. Go to Site Configuration > CDN Integration.
4. In Setting tab, do the following:
Show statistics for the current page: Disabled
6. In tab Basic Mode, enter CDN URL.
Check your sites source code that it is using CDN. Picture links should be replaced with CDN URL addresses.